Here is the configuaration:

DFI 975X/G MB, with 12/06 BIOS (latest)
Intel Core 2 Quad Q6600
Infinity cooler
2x1G Corsair 800MHz member
nVidia 7300 video card, using the nVidia driver
Seagate SATA HD
big honkin' Cooler Master PS >500W
Linux FC6 w all latest patches (as of ~ 1wk ago)

The problem is that the system freezes -- it ranges from freezing during boot, to running for several hours before freezing. It has rarely ran for more than a few hours without freezing. "Freezing" means just that: no crashing or anything, just completely stops responding.

At first, I thought it was the MB, but decided to test it with Windows -- I have currently been running XP in this session for over 6 hours (maybe it is not going to crash).

Anyone here anything about the stability of FC6 (kernel 2.6.19 I think)? Any ideas what might be wrong? I am running at stock speeds. I ran an extensive memory test, and it was fine. I even slowed the memory down to 667MHz, and still got lockups. Early in my testing, I had boosted the CPU to 3G (from 2.4 stock) and at least in Windows (I started my testing in Windows) it ran fine.

Any tips, suggestions, links, etc. would be appreciated.

Your lock ups might be because of your new hardware. Complete lockups are usually a kernel problem more so than other things. Have you tried any other distos or kernels?