The 760 and 780 are both running the Q6600 there with and without the tapemod. That's why I use it. Dell only lists what they offered for sale. Not what actually works. Whether userbenchmark.com is the best or not, it shows what runs and what doesn't, and lets you compare hundreds of combinations at once. When you find an overclocked CPU there it gives you some idea of it's relative performance.
According to Dell my Optiplex 380 only supports 2 core CPUs and 4GB RAM. At Userbench they're running 8GB RAM, and 120W 3.15GHz. Xeon X5460s. Not bad for a $20 computer.