That's socket 775, but is the pre-Conroe days, so unless very lucky, it will only run P4s and perhaps Pentium D CPUs which are essentially dual P4 on one package.
It looks like a P4 651 Cedar Mill to me. Which is decent news as it's at least somewhat likely that that S775 Mobo would run a 65nm Pentium D as an upgrade, and if very lucky, 65nm Core 2 Duo products to some extent.
Anyway, the most likely possibility would be model 915 through model 960 Pentium D, though sticking to 3.0/3.2Ghz makes cooling much easier.
But look for any codes on the mobo itself, or give us a good pic of the board, and I can tell you more definitely what it is and what it's capable of.