The 6990's look to be the best bet as far as max hashing power per motherboard. You should be able to run 3-4 6990's on one computer (with a good PSU).
Using the 6990 is great to concentrate megahashes per slot on the motherboard. But at over $1.00 USD per MH, it can cost twice as much on a per MH basis than other cards. If you are not looking to stuff a single motherboard with as much power as possible, using a few 5830's or 6870's gives you more bang for the buck.