If you upgrade your motherboard, you'll need to upgrade RAM too. Those 630i have DDR2 RAM but anything you buy now will be DDR3 only.
Are you sure you're being seriously held back? That's not a terrible processor.
According to this page you can actually OC your CPU a little, there are step by step instructions. That might squeeze a bit of extra life out of things. If you are genuinely CPU bottlenecked then every little bit of CPU performance will give you extra FPS. http://www.my630i.com/resources/overclocking.html
DISCLAIMER!!! I know nothing about that site or OCing a Dell OEM machine OR that CPU... I can't tell you whether the steps there are legit or how much risk you'd be taking by following them. I'm only suggesting them as it sounds like you're considering upgrading the CPU+Mobo anyway, so perhaps it wouldn't be the end of the world if an OC went wrong on you? /ENDDISCLAIMER