Help with choosing a CPU

Judging from the age of your MB you are looking to upgrade your CPU for perfomence?
If you truly want to make a difference you'd have to upgrade your Motherboard as well I'm afraid.
There are two reasons for this:
1. The CPUs supported by your motherboard are not produced anymore which makes them, well quite expensive in the long run. From a quick look I'd say this is the "best" CPU your motherboard can support:
Amazon.com: Intel Core 2 Extreme X6800 2.9 GHz 4M L2 Cache LGA775 Dual-Core Processor: Electronics It goes for about 80 dollars used and may be found cheaper in some stores but it'd be a lucky find to actually see one still sold in stores.
I haven't really looked too much into Intel Processors lately but as far as I can tell, even the cheapest i5 (socket 1150) would be more powerful than the LGA775 up there.

Upgrading your MB would mean you'd also need a new power supply unit. It'd certainly be a bit espensive, upgrading the MB and CPU are usually the trickiest and most expensive part of upgrading your custom build. But also the two things that last you the longest if done correctly.

2. Again, judging from your MB I'd wager the guess that your graphic card is not really up to date either and might create a bottleneck if you start upgrading ontop of your current setup, especially if you're using the onboard graphics.

But if you're looking for a CPU just to get it running and aren't looking for perfomance that goes beyond the occasional indy game and Office-applications then I'd reccomend the Intel Core 2 Extreme X6800 I linked earlier.