depending on the type of server hardware you may acquire, it may have an equal or greater number of PCI slots. (I would expect to find more 64-bit PCI slots, and/or PCI-X slots on a server).
Keep in mind that what belkins said is right on, you won't find gaming-class video cards on server-class hardware. And ditto for Lasereth, don't expect to see too many AGP slots on server hardware.
The ideal compromise is to find hardware that is in-between, like the Dell Precision line. You can get 15,000-rpm SCSI drives attached to a hardware RAID controller. You can have dual processors, multiple gigs of RAM, etc., etc., and you'll get an 8x AGP slot for your video card on the 360, 450 & 650. Some serious potential there. There's a new model, the 670, which doesn't have an AGP slot but rather a PCI-e slot
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the dell outlet
Of course, if you're looking to build it yourself, you have lots of options.