Yikes...shame I'm not in your area. Otherwise I'd come and give you a hand myself.
For your network cabling:
If you're just running cable in one room...easy enough. Just use plastic tie-anchors along the walls or, better yet, wall-mounted raceways (i.e. <A HREF="
http://www.wiremold.com/" target="_new">Wiremold</A>).
If you're planning to wire your network across multiple rooms, then you should consider setting up a wiring closet or some other wiring center somewhere. The best thing would probably be small wall-mounted 19" racks holding cheap generic patch panels and possibly a shelf or two, then be prepared to do some attic crawling to drop Cat6 cable down within walls. Be sure you know something of the building and electrical codes in your area, i.e. whether you need stuff like grounded metal conduit or firestop material in your walls.
Power lines: BIG deal. Most household power setups would absolutely not be able to handle 20-odd modern PCs across the whole house, much less within one room. Some office buildings would be able to handle it, but check the capacity of your power lines and set up localized circuit breakers for groups of PCs.
I would budget about $500 in parts to wire one room for Ethernet across 20-odd workstations, assuming you know someone who already has the tools (RJ45 crimper, jacket stripper, diagonal cutters, and perhaps a simple ethernet tester). Wiring an entire building for Ethernet would cost quite a bit more, depending largely on the size of the building and the number of telecom outlets per room.
Also, you definitely DON'T need Gigabit running to every PC. 100baseTX is plenty for network gaming clients. The server may need a Gigabit connection tho, since it's going to be serving a lot of clients.
As for PCs...hm. You probably won't be able to get top-of-the-line systems on that budget. I would suggest systems much like the one in your sig--but without the 200GB hard drive, with DVD-ROMs, and with only half the RAM. Maybe a beefier vidcard, like a Radeon 9700 or the like. Don't forget software licensing, either; you don't want every station running off a single home-use license key if the BSA ever comes knocking...
<i><Lionel Hutz> I'll be defending...The SCO Group!!!??? Even if I lose, I'll be famous!</i>