All real, stable OCing (which is what you're looking for) is done through the BIOS, so Step 1 would be getting familiar with yours, also, if you could post what motherboard you have, or write it into your system specs, we could help you much more easily.
a 5200X2 @ 3.0 should uncork any bottlenecks on a 4850, but just barely.
a 5200X2 @ 3.0 should uncork any bottlenecks on a 4850, but just barely.