Go with the X3 720.
You have a chance to unlock the core + for gaming they're the same practically.
If I were you, I'd wait for the Nvidia 400 series and hope to God it causes an ATI price drop.
In which case I'd probably buy a 5770. (That's what I personally would do.)
Case wise, I recommend the Panzer box for lan box cases.
NZXT Panzer is a beast for cooling and space.
Newegg is sold out right now, but Amazon, Directron, ZipZoomFly, and several other online retailers have it.
Also, x8 x8 crossfire or x16 x8 is not bad.
Don't knock it.
Do you even know about bandwidth usage and PCI-E scaling?
techPowerUp :: AMD Radeon HD 5870 PCI-Express Scaling Review :: Page 25 / 26
For gaming, you won't see a major difference in most games?
But there are newer modern games that would benefit from the extra RAM, I think?
( Hardware intensive RTS games mostly )
So if you'll be playing shooters, don't worry about getting 4Gb.