Id personally wait until nVIDIA releases their G92 series of cards next month. I think they are going to be DX10.1.
That killer NIC card is a waste of money.
I have a friend in the UK that just bought a AGEIA Physx card and he said that its a total waste of money. He said all it does is add some particles here and there and gives a guy being shot a ragdoll effect. He returned it for a refund. So I would say that is a waste of money as well.
Arent all Core 2 Extreme CPU's a quad core?
SLi is pointless if you dont have a high resolution monitor of at LEAST 1600x1200. Any resolution lower than that and you are just wasting money. SLi and Crossfire are (again) meant for high resolution monitors that struggle to play games at high settings with one video card.
Get a P35 or better chipset that supports SLi IF you do have a high resolution monitor and you utilize SLi or Crossfire*.
*Crossfire is AMD/ATi's answer to SLi