If you want to overclock, I suggest switching to Intel. You are changing your motherboard anyway. Phenom platform is OK-ish, but new Core Duos are much better. Don't have to go for the top model, don't need to try Quads (which don't really make a difference for games atm, its more for video editing and other tasks, which can generally take advantage of all those cores). Just make sure FSB is not cut down. For example, Intel Core 2 Duo E8200 LGA775 'Wolfdale' 2.66GHz (1333FS

. You can then get it up 500MHz no problem (if you get a decent fan and PSU). Good case recommended too, but not that important since you aren't overclocking much.
4 GB of DDR2 Recommended!!!! Oak, don't be stingy! Get 4GB, you won't regret it! 2GB is a standard for any system, which is running anything more demanding than an Internet browser. Especially now when Vista SP1 is coming out and more games might appear that require dX 10. Mark my words, Oak. 2GB age is at its end. DDR3 is still quite expensive and that's out to work at higher frequencies so its kind of an enthusiast product atm but in terms of size... 2GB isn't good enough anymore.
OcUK is ok, but get a 8800GT with atleast 512 MB of RAM. An alternative for you if you want something newer - buy 2 9600 GT and SLI them. The result is amazing. They run better than all 8800 single cards and 2 of them will cost around 200 quid, so it is a bargain. ATI are shit, don't even bother. They win in terms of power consumption and noise, so if you are building a mid-range quiet system they might be worth it but apart from that... They suck atm. And generally 8800 is a decent range atm since more and more models are coming out, which differ from other cards by more than a sticker on a heatsink.
9800 - don't bother atm. GTX model isn't giving you anything really apart from a few new technologies NVIDIA introducing, like SLI with 3 cards, etc. GX2 is the best card in the world but value-for-money-wise its not good:
1) Noise and heat. 700W PSU recommended.
2) 512 MB RAM per GPU, when 8800 GTX has 768MB.
3) No dX 10.1 support.
4) Hardly any variety on the market, atm.
If you want something as powerful and expensive as 9800 GX2, make sure you wait a bit to get different brands release their cards. They might improve things in terms of noise and heat. Generally GX2 is just a PR move, a show of what nVIDIA chips are capable of (an answer to ATIs dual-processor solution). More model 9 cards are going to be appearing on the market soon and those are the ones you should look out for. I expect them to be much better. And watch ATI too, they are behind atm, but they are picking up the pace after recovering from 2400-2900 failure.