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#11
Here is a fun thread for running Windows 7 on an older computer.
It should work with 512MB of RAM. Normally I wouldn't recommend running an un-matched pair of modules though I guess people do it all the time. In your case it worked fine with Xp and the memory controller should set the memory timing to the slowest module. If both modules work when running separately it may be an issue of running them together.
I wouldn't think temperature would be an issue if it runs XP and passed Memtest.