Asus P6T upgrade!

Hi there fellows :) I'm VERY new here (like 5 minutes old), and wanted to as you guys a question regarding a needed upgrade. At present, the specs of my present PC are as follow:

Asus P6T motherboard

Core i7 920

6GB DDR3 XMS 1300 mhz 3x2GB for triple channel

Gainward GTX 570 1280mb

Hiper Power 780W

1TB Hard drive

Xfi Xtreme Music (which i hate due to its driver problems and cutting and fizz happening randomly)

Games like NFS Most Wanted run quite smoothly everythin max in full hd, with only a few lagging or clipping (don't know what the proper term is) at certain intensive times.

Basically, I was going to get the Corsair Vengeance 2000mhz 12GB set so as to fill the whole RAM slots to get the tri-channel mode again. I think that this should help the clipping and lagging mentoned above.

What else should I get? I'm far from well-off so please offer cheap options :P

Anyone please???