When you say everything, do you mean the system, without monitor, mouse etc?
If yes, you could easily build something decent with that budget. How about this:
CPU:
Phenom II X3 710
Motherboard:
ASUS 780G
RAM:
Corsair 2x2GB 1066
GPU:
HD 4830 <- Bit cheaper compared to the 9800GT and offers the same or better performance.
PSU:
OCZ 500W
HDD: I didn't pick anything here because I don't know how much space you'll need. Just make sure you get something 7200RPM with a SATA interface.
Case:
CM Elite 330
Disc Drive:
Bla
I think that's pretty damn good for just over £400. If you need an OS, go with Vista 64bit.