I would go with a Shuttle xPC. I have 6 of these SSFs and they are very well built. The two I would look at are the SN85G4 (socket 754) or SN95G5 (socket 939) for Athlon64. They both have heatpipe cooling and a 240w PS. The 240w might be too weak for an X800 XT or 6800GT but an X600 or 6600GT should do nicely. You might be able to get a vanilla x800 in there.

I have only seen the x800 XT or 6800GT in the larger P chasis xPC. There might be a "P" chasis design for AMD later on. The "P" chasis is slightly larger ( about 1" higher ) than the current chasis but has a 350w PS and room for 3 drives.
I have 2x SB75G2, SN85G4, 2x SB61G4, SB61G4.
EDIT: Curious myself, I Googled for the SN95G5 and found a review on it. The reviewed system was configured as follows:
http://www.firingsquad.com/har...n95g5_review/page2.asp
Athlon 64 3800+, 1 GB Corsair memory, 250 GB HD, DVD drive, GeForce 6800 GT ( SINGLE SLOT ONLY!! ), so I guess it will run OK on the 240w.
I would probably consider either modifying the vent holes ( drilling more ) or maybe ordering the cover with the larger mesh grille for better airflow to the PSU and video card. I did this for the rigs that have 5900 XTs in them.