I put together a new PC back when Doom 3 came out, but the one thing I didn't upgrade is the RAM, in hopes that prices would go down. It doesn't really seem like they've changed much at all since summer though. Here's my system right now:
Athlon 64 3000+, which overclocks stably at 2.2 GHz
Chaintech VNF3-250 motherboard
Radeon 9800 XT 256mb
Maxtor 7200 RPM SATA HD
And... 512 + 256 MB of PC2100 RAM
As it is, this system runs Doom 3 pretty well and UT2004 beautifully. Haven't tried HL2 on it yet. I do wonder how much of a performance jump I would get with a gig of actual PC3200 RAM.
Edit: Forgot the motherboard.