Yeah, some spare batteries (get them from www.sterlingtek.com.
Only $11 for an 1800mAh BP511 compatible battery!) a couple 4GB CompactFlash or Microdrives, a 400GB HD for your computer, and a faster processor to handle the RAW conversions! hehehe
Thing is, I'm only half-joking with the above. I have two 4GB Microdrives that I used on my 20D, and I was able to get a little over 500 images on one card. On the 5D, I'm lucky if I get 250. Obviously it's to be expected with the jump in resolution.
Converting RAW files from my 20D used to take roughly 6-8 seconds each on my Athlon64 X2 4800+ system. It jumped to 16-18 seconds each on the 5D. I'm also running out of space on my hard drives for the files too! LOL!
All the same, I'm in love with my 5D! This camera is incredible! You're going to love it!