Do not siwtch your RAM to Corsair XMS, G.Skill is just flat out better, TCCD is the best memory you'll find and Corsair can't hold up against it.
Also, switch your board to a DFI Lanparty nf4 SLI-DR board, DFI is by far better than Asus and it is widely agreed that the Lanparty is the best socket 939 board on the market now.
If you get a 6600GT now, save up some money, you can use the eVGA step up program which basically allows you to trade in your existing card as a "coupon", eVGA will basically slash whatever you paid on the 6600GT off the price of the 6800GT.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w