Lot's of helpful advice thus far, but I'll add a little something extra.
As far as your memory cards go, I would be wary of using cards upwards of 8gb, unless you're using them in the second slot of the mk3. I shot a wedding this past weekend with my mk3 all in RAW and you'll average 270ish pictures per card. Now, that's not a lot, but say you get a 16gb card, take 540 shots, and then something happens and the card breaks (God forbid)? Not that it is very common, but random things happen to cards sometimes and they just don't work. I wouldn't want to risk it.
Here's my suggestion. I shoot with four 8gb SD cards, and have them attached to my belt in a small holding case. Before a card fills up, I can simply swap it out in 10 seconds or so. Now, those ten seconds may be crucial, so you really need to judge how many shots you have left. Wouldn't want to be caught with fifteen pictures left at the kiss haha.
And as far as the cards, I'm using the Sandisk Ultra and Extreme, both 30mb/s, and unless you plan on taking more than 14 or so RAW pictures on continuous shooting, they will definitely be fast enough to record what ever pictures you're taking. Just got two more from Best Buy the other day for $12.99 a piece. That sounds cheap, but I've been using two of the Extremes for over two years now and have never had a problem.
Also, take a flash. It doesn't matter how well lit the venue is, it is always helpful to have one just in case. I see a lot of people saying it's great to shoot at 6,400 ISO on the mk3, but from personal experience with mine, I wouldn't push it that high at a wedding unless I absolutely needed to. The less grain you can keep out of the picture, the less work you have to put in in post.
Hope that helps.