Upgrade time £600 to spare

[S-H-A-N-E]

Just finished paying my credit card off , i have £600 to spare for upgrade

My system

Viewsonic VX912B - Replacement around april ,
AMD FX 57
Asus A8N-SLi SE nForce4 SLi
Leadtek GeForce 7800 GTX 256
2GB PC3200 DDR (400MHz)
Creative Sound Blaster Audigy 2 Platinum - only use headphones

I want to upgrade to 7900 512 but not sure which version off card any idea`s

Any idea`s also for new pc case with x 2 120 mm fans