If you are going for the XPS420, and are not too bothered about the gaming side, you could save yourself around about £100, by going for the E8400 cpu, and the 9600/8600 graphics card.
As for ram, 8Gb is only advisable if you are getting a 64bit operating system, which may cause problems with peripherals and/or software.
A 32 bit system can only see under 3.5Gb ram, so in my opinion 3Gb is sufficient.
P.S. the 8800GT is one of the fastest cards available, and will run most if not all games with impunity.