I just built my daughter a box out of stuff I had laying around, runs SOD really well. AMD Athlon XP 3200, ATI X800 Pro, 1 gig of RAM, WinXP. Runs great, chews through the game no probs.
If you are talking about building something NEW, however, you can do so for less than $500 if you value shop. I just built myself a new box no too long ago (3-4 months). AMD 5000+ BE, ATI 3870, MSI mobo, 4 GB RAM. Admittedly, I already had a case, HDD, Optical drives, monitor, and peripherals. But still, all those new parts are < $100 US save for the Video card, which is now ~ $120 US.