Ok, with a socket 775 CPU, a PCI-E vid card, and regular DDR RAM, you are limited to the i915 chipset mobos that use regular DDR RAM (a few use DDR2 RAM). Unfortunately, they aren't the best overclockers. For example, the Abit AG8 which I had at one time, it was a nice stable feature rich mobo, but would only O/C to about 240 FSB. Perhaps that would be good enough for you? That would "probably" allow a good 3.2 to hit 3.8 gig (assuming you have good cooling).
Sometimes, 4 sticks of RAM will hold back an O/C a little. Perhaps you should think about selling your old RAM and investing in 2X512 DDR2 RAM, then you could get a better overclocking mobo, like the Abit AA8XE (which I have) or if you want an upgrade path to the dual core Pentium D 8XX series, one of the i945/i955 chipset mobos like the Asus P5WD2 or Abit AW8.