If you want to splash out a little to keep it up to pace, but not as far as to get a new CPU, then I would suggest the Hyper 212+. Great cooler with solid performance at all RPM's. With a Hyper 212+ and a good thermal paste like Arctic MX-4 for example, I'd suggest about 3.9-4GHz on 1.33-1.35 V range. If you want to push the limits a little more, 4+GHz at 1.4xx V range would be pretty obtainable, but while it may be stable don't expect it to last. The lifetime of the CPU will dive down after 1.4V, at least the Phenom x4. With stock cooling... hm, probably stick with 3.5, but you could maybe go with up to 3.7GHz, on the 1.33-1.35 V range, which should suit fine. However, that boils more down to what your general ambient and case temperatures are. Naturally enough, you don't want to OC very far, if at all with stock cooling if your ambient temperature is about 30C. Let me know how you go.