Edit- In reading the forums, it looks as if Eyefinity defaults to the lowest resolution if one mixes monitors with different resolutions. So this means I could not run my 30" at its native 2560x1600?
This is true in eyefinity mode. But if you're using it to play movies, why not just keep it in extended mode while in desktop so you can have all your resolution. You could play games in windowed mode, but i dunno how much power you'll need to do that. This is why I've decided to go 3 30". It's more expensive but remember you can always play games in 3x1920x1200 (5760x1200) as well, which people have been managing on a single 5850.
EDIT: Also if you you decide to go Triple 30" I'd say buy used. I'm finding great deals on craigslist, I got my initial 30" for $400 and I'm going to get another today for around $500. Ebay isn't so good, they tend to have there 30" monitors $800+