The maximum you could possibly upgrade your CPU is a S754 3700+ (judging from the motherboard specs click here). Not only is this CPU unavailable to buy, but even if you manage to get one (off eBay for example) it's going to make little difference.
These days, almost all CPUs are dual-core, so it'll be difficult not to go for one.
If you're going to upgrade, you might as well upgrade the CPU, motherboard & RAM to make it worthwhile.
Have a look on CCLOnline click here & Novatech click here for bundles. It can be either AMD / Intel, but the motherboard needs to be mATX so it fits in your case. Also check if your PSU has the required 24-pin power connector (20-pin won't do). G