Occasionally, in the midst of playing Empires, the game will crash and freeze the entire computer, requiring a 5 second power button restart. It doesn't do this for any other Source games as far as I know, for example, it has not yet happened in Counterstrike. I may want to run CS:S just to test. What system specs should I post?
Windows 7 Home Premium SP1 32 bit AMD Athlon DualCore 4050e 2.1GHz 3GB RAM NVIDIA GeForce 8400 GS How do I put up the driver info?
typing dxdiag into the Windows "Run..." dialog (for Vista and 7: the "Search programs and files" field when you click on Start) should net you a lot of information.
Can't really see any possible issues. Make sure you have the latest drivers of everything. Perhaps have a look at crash logs in the Event Viewer (same way you did dxdiag, only now the command is "eventvwr.msc"), and download RivaTuner, which allows you to monitor your CPU and graphic card temperatures.
Most probably a problem with the graphic card or the power supply. Can you play other games that max out your system without problems?