With your parts, you should be able to hit at least 4.5GHz with turbo, but still, you really should start your OC the right way - by raising the multiplier in small increments (like 5, or so)until you reach the point where Windows doesn't load. At this point, you would step back your mutli by 1 until Windows does load.
Once you've found a multiplier that allows you to load Windows, test your system's stability with Prime95, a free downloadable software. Run Prime95 on Blend for about 30 minutes to an hour.
If your system is stable, you can either leave it alone, or you can try some of the advanced OC steps by adjusting voltages other features.
ATM you won't gain a lot of performance increase from overclocking the CPU in games, but you will in benchmarks and editing software. Overclocking the GPU though will give you realistic, real life gaming performance instantly. This can be done within Windows (much easier than setting PCI-E FSB speeds and stuff in BIOS) with something like MSI Afterburner (works with any graphics card and any GPU). I'd recommend overclocking the GPU first to around 850MHz, with a 50mV increase on the VCore. If temperatures remain stable after a benchmark test (with MSI Kombustor or 3DMark, max load temps less than 80'C) you can go higher, in 25MHz intervals and increasing the VCore with 35mV every time, until it becomes unstable or the load temps reach about 85'C ~ 90'C (absolute MAX). I'm guessing you'll be able to hit around 900MHz with that card, easy temps and stable voltage.
The extreme "upgrade" for the HD6950 2GB though is flashing the BIOS to that of the HD6970's. It's free, relatively easy and provides superior 6970 performance at a cheap price. A very important thing to remember though is that you lose the warranty when this is done.