(which kind of gfx cards) want to run 4 monitors...

DVI to D-sub splitter would not work, but if your card supports dual-link DVI and signal separation, then some dual dual-link DVI card could use a splitter dongle to give you 4 single-link DVIs. These cards have speciallized connectors and dongles thoush like DMS59 and VHDCI connectors. There are some low end (but expensive) GF6 and X1K series cards that can support 2/4 monitors this way, but they are about as expensive as the professional solutions like the Matrox, and ATi MireMV, Colorgraphic Xentera, and nV Quadro solutions.

Probably the Cheapest and easiet solution would be the Gemini style cards wit 2 VPUs and 4 outputs. They are suposed to be out shortly, and should be about the $200-250 mark.

If you can find one of the dedicated solutions for cheap the that would work, but if you've got the space try for 2 sinbgles card, that would be the cheapest way, especially for stuff like that that doesn't need heavy duty output.
BTW, you can find some small boards (not sure mini/micro) tyhat have integrated DVI+VGA/Video giving you 2 outputs from there, and then you can add the card to some of them and not have to disable the onboard graphics.

I personally would suggest getting a pci card along with the main video card and you would be able to run more than 2. Here's a link to a topic talking about the same sort of thing... http://www.tomshardware.com/forum/ [...] 50-express

Another solution, if your cousin is willing to take step down in cpu power would be getting a socket 939 mATX board. I saw one that had SLI and was mATX. I forget the name of it, but I know it was out there. (and newegg at one point carried it.)