It depends. Since LGA 775 is quitting, there won't be easy to find a suitable processor.
If u could tell us the budget it would be help. If u decided to keep this mobo, u should find a high-freq dual-core/quad-core CPU and a good gaming GPU. (And a good CPU cooler if u want OCing)
Here are some CPU candidates for u:
Pentium E5700 3.0Ghz, $60 at ncix.com
Core 2 E8300 2.83Ghz, $68 at superbiiz.com
Core 2 Q6600 2.4ghz quad-core, $97 at superbiiz.com
Core 2 Q8400 2.66Ghz quad-core, $128 at superbiiz.com
going up the price will soar, too. These CPUs are aging but still has good overclocking potential and still performs well, too (Not as good as 2rd/3rd i3/i5/i7 CPU but still strong)
And check carefully if these CPU is refurbished...