I spent 280 bucks to update my old athlon thunderbird 1.2 to a phenom II 555 black edition cpu at 3.2 ghz.
All I changed was cpu, motherboard, memory, and power supply and got a huge performance boost while also removing all expansion cards.
Motherboard with integrated graphics.....Asus M4A785-M.......$79.00
Processor................phen omII 555 black edition...................$89.00
2 gigs of memory......(xp only recognizes 3)..........................$27.99
550 watt power supply....................... ............................. ..$39.00
I did add much larger cpu cooler with quiet fan.......................$29.00
I just used the legacy ide port for the cd and hard drive and, though it reduces disk performance greatly, it runs the game fine.
The graphics is a weak integrated ati HD4200 but it plays the game fine at 60fps.
Of course.....assembling this and making it work could open up a huge can of worms fiddling with drivers and settings as about 50% of builds/upgrades seem to do.