I'd opt for a lightweight distro. I'm running a desktop with the same specs, CPU and RAM-wise as you, and am more than happy with Anti-x. I've not had to re-install once, (used it for over six months), and have everything working as I like. It has a small and user-friendly forum, and makes use of debian-based repositories - in other words lots of applications for you to try.
I wouldn't recommend running a KDE4 desktop - which is distinct from the distro - as it will likely be a menace to your CPU, and be too slow.