I got Optiplex 755 that I've been using as a homer server (running Debian 7, no gui). It currently has an Core 2 Duo E8200 2.66Ghz, 7GB RAM, and 2 2TB drive.
no gui will definitely free up some resources
I'm thinking about using proxmox, so Im wondering if it'll be worth it to throw in more RAM and upgrade to a Q6600/Q6700. Performance wise
not to confident with this...
1. RAM: When dealing with prefab machines, take care to research exactly what the mobo will support. It is probably an 8GB max
2. CPU:
http://ark.intel.com/products/33909/Intel-Core2-Duo-Processor-E8200-(6M-Cache-2_66-GHz-1333-MHz-FSB)
This is the 45nm generation. Again, going to a Q6600 or a Q6700 is a step back to the 65nm generation. And the same rules apply when dealing with prefab machines. Dell, HP, whatever....will place tighter rules on what hardware can work in their boards.
I recommend staying in the same gen cpu....IIRC the Q9400 will bascially be two E8200's on one package.
with what you're doing, at MOST I'd go with the 2x2TB drives because those can be move to a new machine. The RAM and the CPU...well, research it well, and if you're spending more than $100 (IMO) then you may as well save the money and just use what you have for now. The good news is that most *nix installations don't take too much resources, makes them great for VM test runs.