I think the 64" white umbrella without the black outer cover will be fine. The outer cover makes the reflecting somewhat more efficient.
I would still use it in REFLECTIVE mode rather than shoot thru for this event. You'll have plenty of power with the Einstein, anyway. On the off chance that you have to position the umbrella close to an oddly colored wall, take a simple fabric (a small cheap black curtain from Target) just to cover the outer side of the umbrella (a couple clips will hold it). This is unlikely to be needed, but if you were say up against a bright lime green wall, the portion of light that passes thru the umbrella and hits the wall could reflect back a bit lime green color cast on people nearer the umbrella. (This is why a black cover is a good long term investment--for example, if I want to shoot a quick portrait in a living room at someone's house, the walls/ceiling are close, and they can act like being Inside a colored softbox. Controlling spill is important then). In this venue, I doubt this will be a big concern.
So, two 64 whites, and another Einstein will certainly allow you to cover a bigger area, and seems reasonable to me. Otherwise you might only be able to capture images in one corner.
You can get lightstands with wheels, so you could periodically "migrate" your setup (very carefully) but this would be undesirable, prone to a multitude of mishaps. BTW, get some sandbags for your light stands (Cowboy Studio via Amazonhas a pretty good deal, and a couple bags of sand from Home Depot fills em up:
http://www.amazon.com/gp/product/B001AHALEC/)
You'll want to consider safety at the shoot, both for participants and for your gear :-) The sandbags will help prevent your lightstands from getting knocked over. In addition, I would use some bright safety orange or safety yellow duct tape: tape the ends of the legs to the floor, put small strips up the legs for visibility. It is really easy to trip over them, especially people unfamiliar or just otherwise engaged. You might even consider some small orange safety cones.
When using power cords, be sure they can't be snagged or tripped over. The cord from your Einstein down the stand should be kept close to the stand, not dangling where someone could hook it with an arm, etc. I use some orange plastic power cord clips from home depot, as it puts a little more orange on the stand for people to see and avoid. When the cords reach the floor, and particularly with a routed extension cord, get it over to a wall and follow a baseboard to the outlet. Tape it in place at several places along the wall. Wherever the cord is on the floor, cover it completely with brightly colored safety tape, and choose your lightstand position so that it is out of the way, and the cord is not in any place that might conceivably be used as a walkway by an inebriated person or a child :-) Don't try to blockade around it with chairs, as you are inviting people to sit there, lean or grab your lightstand, or they will go grab a chair to move somewhere, and thwack your lightstand good.
In general, while the attendees at the event are your subjects, friends, customers, when they approach your gear, they become unintentional threats. Divert, prevent, protect.
I'm assuming, but do not know, that you have a speedlight? Your plan to light the room in general ways is good for the general coverage, but you will, I assume, also need to get certain moments (cake cutting, toasts, first dance, etc.--btw, meet with the couple in advance and create a list of the must-have shots, take it with you as a checklist). Scout the venue, know the planned layout, and select your lighting locations so that those events are able to be lit (maybe you have to swivel the light a bit at a certain time). Plan to be surprised--when you get to the actual event, things will not go as expected. Tables may be arranged differently, etc. But if you have planned ahead, then you can recognize what has changed, how it might affect you, and adjust accordingly).
If you can enlist a volunteer second shooter with a speedlight (who knows what they are doing), that is a good back up for roving around, and for getting detail shots, special moment shots.
If you can have a friend/family member be a "gopher" that can be very helpful. They can go find someone who needs to be in the shot, go grab your gear bag if you need something, keep tabs on what is going on (like come get you if the cake cutting is about to start suddenly).
You can ask the wedding party to help, too--you can ask a groomsman or bridesmaid to go find a 'missing person', etc.
Scout the venue, plan where you will set up, with back up options (tables or a wet bar or a dj may be set up where you planned to, check with the couple for plans if possible).
In general, think redundancy (within your budget, of course): if you have a second camera, take it. Take enough extension cords even if you have the vagabond. Take extra batteries for your cameras, pocket wizards. Have extra lenses. Extra memory cards. Have your speed light with you, even if you don't plan to use it. With extra batteries. Imagine variations on the plans: be ready to shoot outdoors for some shots, be ready to shoot with only available light indoors if need be: there may be a killer shot in a hallway, be ready to goose ISO, go full auto, or whatever works on your camera to get that shot, and then know how to quickly get back to settings for strobe use. If you have a point and shoot camera, drop it in your pocket. 99% of the time, you won't need any particular back-up item, but when that 1% situation occurs, it will make the prep more than worthwhile :-)
You have some time to do google searches, get a good book or three, and pick up both do and don't pointers. Preparation will diminish surprises and stress, and you can enjoy yourself a lot more at the event. (I had a friend who had never shot a wedding get called on a Friday evening by a friend who's wedding photog had cancelled last minute--so she went and shot the wedding on Saturday afternoon--talk about stress! Her email signature for several weeks afterwards was "never shoot weddings!")
Also, keep in mind: I have not shot a wedding. I have done events, though, and I have used Einsteins, modifiers, speedlights, etc. So most of my comments are fairly generalized. You will definitely want to get some books specific to wedding photography. Jasmine Starr did a live wedding shoot at Creative Live as an online web class.
http://creativelive.com/courses/jasmine_star I have seen part of it, could be worth watching, but it might not be worth the $ to you. I am sure there are other options out there.
Again, Good Luck, and Have Fun!