Today’s video cards like the nVidia 9800GTX or the 280GTX cards are humongous! I mean huge! Literally, they are massive. They take up a lot of room within a regular sized ATX case. If you were to put two for SLI, you’re at a lost for a lot of space within a regular sized ATX case.
Here is a picture of an nVidia 9800GTX. The card is as long as the box is!
If you plan on buying one of these, make sure you have:
A very beefy power supply Something along the lines of 500-600W and up. You will have to make sure there are two (possibly even four) 6-pin power adapters for your video card. This 9800GTX pictured here takes 2×6-pin PCI-E power connectors.
An extremely beefy/full sized ATX case If your case is rather old and you had maybe an nVidia 6800GT or maybe even something older than that, then you SHOULD upgrade your case as well as power supply alongside your new video card. Maybe perhaps an Antec P182 will do well although I hear that it barely fits all of the cards along with the SATA cables. Do your research before you buy!