So, I'm sticking with my perfectly good 366 MHz laptop until the cheap-o boxes can handle Doom-3 and kin, which will probably be in about 3 months.
Did I just hear a glove hit the ground? Really the only thing holding down the above 3000 based machine is the lack of an agp slot for a better video card.
However, try this:
http://configure.us.dell.com/dellstore/config.aspx?customer_id=04&kc=01240&oc=s20903f&x=2&y=10
Pentium 4, 2.8GHz, 256MB ram, 40gb hard drive. Technically a server platform but good to go as a workstation as well. Need to bring your own OS as one is not included. As configured, $349 after $100 rebate. VERY expandable as this is intended to be a small business server and can handle RAID, SATA, plenty of memory and disks. Make sure you select the free keyboard and mouse options, they're off by default.
Add this to it:
http://www.newegg.com/app/ViewProductDesc.asp?description=14-102-331&depa=1
9600xt agp card, $163
Or better still, one of my current very favorite cards:
http://www.newegg.com/app/ViewProductDesc.asp?description=14-102-352&depa=1
the all-in-wonder 9600xt, $189
The all-in-wonder version brings a high quality tv tuner, fm tuner, encodes and decodes mpeg2 in hardware, has the new remote wonder 2 rf remote, supports dual monitors, and runs both the core and ram at a slightly higher clock rate than the base 9600xt. Oh yeah...use the s-video out to put it on your big screen or, like I do...to your infocus projector for a 10' picture on the wall.
Both will run doom 3 at 1024x768 in "high" quality mode at very reasonable frame rates. Higher resolutions or quality settings buy you no visually perceptable enhancements. Off the top of my head I think you'll get about 25-35fps in this mode, and 60+ in 800x600 "medium" mode. The current ati catalyst 4.9 drivers have doom3 optimizations with more planned for the next couple of releases. Haunting a few of the doom3 enthusiast sites will also give you some great doom3 .CFG optimizations for specific cards. Add another 5-10fps@1024x768 and 10-20 in 800x600 with full optimizations.
Roughly $500 for a machine that can be topped on paper, but probably not in real use.
Half-life2 and Halo2 are both supposed to have more efficient engines than doom3...
Kick butt machine for video work as well.