Sounds like a bit of a bargain really, at £500 (inc?) plus another £200 for the monitor.
What I feel people would comment on was the relative benefits to be had in spending an extra £100 for a 17" TFT monitor (if TFT screen is what you require). The additional viewing size is really much better and the AOC monitor from overclockers.co.uk amongst others is a very good monitor (as Djohn would testify).
The other factor to note would be the graphics card. I have used it briefly and it is not really the best 3D graphics performer, though it does hold it's own for light usage. If looking for a budget solution, go for the MX600 instead, slightly more expensive but much better. Or, if you can stretch the budget a little further, go for the GF4 Titanium version, the '4200' (GF4 Ti4200) model which is really the gamer's graphics card, costs almost double the MX400 though at say £120 inc. It performs at double the ability of the former MX400 card also though, but is only really required if the system is going to get a lot of use gaming wise by a serious gamer, really. Only the latest most demanding games require such processing power - these are targeted more at the late teens/20/30 somethings (and Ordep ;o]).
Otherwise - a great system it seems. The P4 will run more silently than an AMD Athlon system, as it requires less cooling so perhaps a good choice, though Athlon systems are tradionally less expensive than Intel Pentium chips. A link to the computer might encourage more opinion - good luck all the same with it though.