OK, so I've found me a nice Pentium 4 (don't really know what kind yet, haven't even plugged it in yet) ThinkCentre and I'm thinking of pimping it out with an old X1900 card but I don't really know where to start. Business desktops are usually a pain in the butt to upgrade for gaming/entertainment uses, and I'd like to know if you guys have any experience with this, and also I'd like to know whether or not a P4 processor would "hold back" the performance of a X1900 or even an 8800 GTX.

If the cards exist in a low-profile model (about half the normal width of a PCI card, with a bracket to match), I don't see why not.

Cool, I have another Dell Dimension that's slimline that could do with an upgrade, since it's going in the living room a slimline might be a bettter fit.

Yeah, the only trouble would be to actually FIND a low-profile model of the card - also hope that the computer comes with an AGP or PCI-E port and not only PCI.
A normal-length bracket can always be fixed manually, by removing it from the card and bending it at the right point to match the desired length.