|You're in pretty good shape for processor and memory.|
230v is the input voltage of your power supply. You're interested in the output power and all that should be labeled on the side of the power supply. It'll say something like 350 watts and give the specs of the +3.3v, +5v, and +12v outputs in amps. Let us know what info is on the psu label.
As for cards, I think you should try to find something for less than £50. No need to make a money pit out of an old pc.
The page I linked to has a couple of X1650Pro cards for about $50US, an HD2600XT for $80US, and a HD3850 for 85 bucks. You might also look for a X1950Pro.
The HD3850 is the best AGP card out there but you may also need a new psu to run it.
Yeah, if I was shopping in the UK, I'd sure be looking to keep the cost down too. You might shop for a X1950Pro for < £40. Maybe some of the UK guys here will chime in with other ideas.