Just finished building a decent Retro Rig from this period, consisting of ...
Intel Pentium Dual Core e2160 @ 1.8Ghz stock
4CoreDual SATA2 Asrock 775 motherboard
AGP 8x : Radeon x800se / Geforce 6800xt / Leadtek 5900 Ultra (tested only these so far)
1GB (2x512) OCZ 433Mhz DDR
Audigy 2 ZS PCI
40GB Maxtor IDE 5400rpm
74GB Western Raptor SATA 10,000rpm
DVD/RW Optical SATA
500W Apline PSU
Asus brushed steel black ATX case (made in 2004, 211mm wide)
Windows 2000
The cheat is the e2160, being that it was made in 2006, but as it only runs at a stock 1.8Ghz yet is comparable to a Prescott P4 @ 3.2Ghz in gaming performance, I picked the e2160 because it runs MUCH cooler. Also has the option to overclock to silly speeds, not that I am considering doing this. I picked the DDR 433Mhz over using DDR2 because the OCZ stuff looks great with the RAM coolers. Also wanted to use the AGP 8x port instead of the PCI-ex 4x, because I have lots of AGP cards from this period, and my slowest PCI-ex card is a 1950XTX, which is overkill for 2004 as it can score over 20,000 in 3DMark 2003 with a decent CPU.
I've ran a bunch of synthetic benchmarks with all 3 cards above. Still have a geforce 5700v, 9800pro and 1950xt to test. Also ran Doom III for a quick blast to try each card, but only at 1024x768. Even the 5900 Ultra managed a playable framerate but nothing as smooth as the x800se or 6800xt.
Performance in 3DMark 2000 and 2001se were pretty much similar across all cards (obviously CPU bound), with the 6800xt winning by a slight margin in 2001se. The big differences were in 3D Mark 2003...
5900 Ultra - 5752
x800se - 8929
6800xt - 11526
I5-2500K @ 4.0Ghz + R9 290 + 8GB DDR3 1333 :: I3-540 @ 4.2 GHZ + 6870 4GB DDR3 2000 :: E6300 @ 2.7 GHZ + 1950XTX 2GB DDR2 800 :: A64 3700 + 1950PRO AGP 2GB DDR400 :: K63+ @ 550MHZ + V2 SLI 256 PC133:: P200 + MYSTIQUE / 3Dfx 128 PC66