The best value for a P4 right now is the P4 2.4c (800Mhz FSB), which overclocks easily (for the most part) to on average around 3.0Ghz+, I know it's around $175USD but not sure how much it is over there where you are. Try shopping around online for the best deals. I personally prefer Abit mobos, since they offer easy overclockability (is that a word?

)and stability. I personally got my Abit IS7-E for $99USD, with 8x agp, dual channel capability, onboard sound and LAN. (although to be honest, the onboard sound is to be desired....maybe you might wanna shell out a little extra and get a basic sound card)
For sub £800 (~$1200, correct me if I'm wrong) you can probably get a Radeon 9700 pro, although if it gets tight you might wanna knock down the video card to a 9500 or 9600.