O dam, I didn't know the 6850 was only $179!That's a schweet deal!
thinking 6850 (thx unforgiven 4 link)http://www.newegg.com/Product/Product.aspx?Item=N82E16814131374&cm_re=6850_radeon_hd-_-14-131-374-_-Product$170 after MIR
Is the 4850/sink loop compatible with the 6850?The cooling mounts are usually the same spot. If the sink doesn't bump into any ram or capacitors, could easily be just a swap.
I think your CPU should be enough with that card (for gaming). I wouldn't worry too much about overclocking yet until you see a lack of performance.I used to be like Tarsier, and overclock my machine so notepad would be super fast, but now it's on a need basis.