What do you need for Quad-SLI ? A pretty beefy system, that's for sure, here are your ingredients:
Not one but two times a GeForce 9800 GX2
nForce 680/780/790 mainboard
2 GB memory
Core to duo/quad processor - 2.67 GHz or faster.
Windows Vista fully patched and updated (SP1 recommended)
Quality 850 Watts or higher specced power supply
High resolution monitor (1920x1200 or above)
Positive thinking and a bit of luck
Games man !
Quad SLI Review With Dual 9800 GX2
EDIT:
Im really sorry, i just realised that this is prolly in the wrong thread.