New stuff keeps coming out, pushing prices down. I just built a Shuttle XPS system (lots of USB and eSATA for expandability) with a 2.5ghz Core 2 Quad, 4gig DDR2 Dual Channel memory (can expand to 16gig), a 500gig 7200rpm SATA drive (room for one more), DVD-/+RW, 23" WS HD LCD, and a copy of Win 7 Pro x64 for right around $800. If I had gone with a standard MOBO and case, I could have knocked nearly $100 off that price. Going with Linux would have saved $149.
BTW, this was the fastest and easiest Windows install I've ever done. The Win7 disc had every driver I needed and the system went from bare drive to me online in 20 minutes. Really. I timed it. I had to switch the Firewire driver to a legacy one, but everything else worked out of the gate.
Chris