I know everyone says to buy the components and then build it, but big-name pre-built PCs are REALLY cheap now (kind of like the low mortgage rates!). I just picked up a Dell 4550 configured as such for $589 (plus tax) from the Small Business Site.
CPU: PIV 2.
53 Ghz, 533 MHZ FSB
Memory: 256MB, 333 MHZ (I'm going to double it from Crucial.com for ~$40)
HDD: 60GB 7200 RPM
40x CD/RW
Windows XP Home
No monitor
No Office SW
1 year parts/labor next-day-service
No Modem
No Floppy
16MB USB Key drive (or whatever its called)
If you go with Small Business, you can get away with the 1 year warranty, no speakers, no modem, which saves you some money.
Now I haven't priced a similar PC "A LA-Carte", but I couldn't pass up the $589 price to save me the hassle of picking individual components.
The PC has served me well in the ~1 week I've had it. I have N-Track, a Delta 44, and Waves plug-ins all playing along nicely so far.
Hope that helps and good luck!