I was bored, so I went on New Egg and selected parts for what I'd would use for a new, lowish budget CAD machine. This doesn't include a monitor, total price is in the lower 800s, but the processor could be downgraded to an Athlon 64 3000+ for $180 savings. (The 3000+ is still nice, my brother's computer has one and it can play Quake 4 with nice amounts of effects with no problems) Anyway, Here goes.
DFI DAGF ATX Socket-939 Motherboard $82.00
BUFFALO Select 2GB Dual Channel RAM $146.99
eVGA Geforce 7600GS 256MB PCI Express x16 video card $114.99
ASPIRE X-Dreamer II ATX midtower case w/ PSU $56.99
NEC multipurpose DVD/CD burner $33.99
Western Digital Raptor 36.7 GB 10,000 rpm SATA hard drive $101.99
AMD Athlon 64 X2 3800+ Dual Core $297.00
That might be a bit out of your price range, but various parts (RAM, proc, hard drive, video card) could be downgraded a bit.