I'm sitting here with some G.Skill 4GB DDR2 PQ PC2-8000C5 (2x2GB) CAS5 (£82) ready to be opened however since ordering the RAM a few people have said I don't need PC2-8000 RAM and should buy some GeIL 4GB (2x2GB) PC2-6400C5 800MHz Black Dragon DDR2 (£67) instead.
This would save me about £15 but I'd have to wait until tomorrow (not long I suppose) before it will arrive and I can use it.
I will only be overclocking my CPU to about 3.2-3.3GHz as I want my hardware to last about 15 months before parts start dying. I don't wish to overclock my RAM. From previous experience RAM dies very easily.
Since your going to OC either would be fine. The DDR2 800 mHz (6400) stuff should be fine for 99% of situations and will allow you to OC to slightly above 400mHz FSB. That should get you up to about 3.6-3.8 Ghz (theoretically - 9 x 400mhz = 3.6 ghz). Having the higher FSB DDR2 will make things easier, but if your wanting to save some $, you could get by with the 800mHz stuff.