Quake4 and COD2 can play stable for hours and hours without any problems on a properly configured system with properly functioning hardware.

If the system is locking up, especially if a reset is the only way out, this is absolutely a hardware or hardware configuration issue. This would also encompass temperature or power issues.

The best first steps are:
1) Update to the latest BIOS for your mainboard.
2) Update to the latest chipset drivers. I know it says in the original post LATEST Nforce drivers, but the ones on NVidia's website are old, crusty and do have some issues. I'd hit www.guru3d.com and get some of the newer ones there if you haven't already.
3) Update to newer and beta Forceware drivers if NVidia. Get the 5.11 Catalyst drivers if ATI.
4) Relax timings in the BIOS- for memory, cpu and other hardware. Set AGP to 4x, then 2x for a test. Disable Fast Writes.
5) Download (google) memtest86, make a boot floppy and run this overnight for any possible errors.
6) Google/install hardware monitors to monitor cpu/system temp and all voltage rails during gameplay. One that logs is critical as we'll need to inspect this during a lockup.

Any foreign software, tweakers, systray tools or drivers should also be uninstalled. Things such as internet security, systray toys, color/desktop items (like Windowblinds or whatnot), 3rd party tweakers, DVD/virtual emulators, memory managers, color correction translators, video refresh tools, etc.etc. Many of these actually need to be UNINSTALLED rather than disabled in order to properly isolate if they are the culprit. I'd lean more towards temp, BIOS/hardware config, drivers and/or power for a lockup that requires a reset though.

Good luck!

have u tried using Kx audio drivers and disabling the eax in the games configuration ? i had problems consisting of that repeating sound and i later to have found out its the drivers poor support for newer games , i now use Kx audio driver for my sound card which is a audigy 1 and my problems seem to have disappeared but if you use them drivers u cannot use the eax in the game as it will cause the same sympoms , and also using them drivers ( Kx ) i found out i am able to support more sounds then from the actual driver themselves . now a lil sample test to do , remove the sound card use the on board audio and then run the game and see if any difference taken place .