2 graphics cards?

You must have 2 PCI-Express slots.

if you are going with nVidia you must have nVidia SLI Ready motherboard & If you are going with ATI you must have ATI Crossfire Ready motherboard.

You must have same graphic card (2 7900GS cards) ,same memory(2*256MB memory) (maybe different sizes possible.But I'm not sure), different memory speeds ok (First one 500MHz & Second one 450MHz)

different Brand names ok (Asus & MSI) & a good power supply (550W maybe)

2 pci slots? the only thing in my computer really is the graphics card, does this mean i will be able to plug them both in?pete_merlin

Some motherboards has 2 PCI-Express slots, some has only 1 PCI-Express slot.

So you have to look your motherboard theres 2 slots.

1 is used by your current graphic card & look for another one same like that. 

[QUOTE="pete_merlin"]2 pci slots? the only thing in my computer really is the graphics card, does this mean i will be able to plug them both in?LahiruD

Some motherboards has 2 PCI-Express slots, some has only 1 PCI-Express slot.

So you have to look your motherboard theres 2 slots.

1 is used by your current graphic card & look for another one same like that.

Some even have 3 or 4 pci-e slots btw... just an offshoot there :) 

SLi somtimes isnt worth it you might only get 10-20fps more at max and some games dont use it... I dont reccomend itLouieV13

Very True rather get a a DX10 card like a 8800GTS 320 or 640 Mb