Agree with above^^
SLI-DR (£140ish)
XFX 7800GTX EE (£365)
AMD64 FX-55 (£495)
or:
AMD64 X2 4800+ (£455)
I would go for the second, put under the mach and perhaps spend the extra £50 on upgraded ram, dunno what you have atm, some nice mushkin at a certain well known retailer atm