Honestly.... you can save a lot of money or have a very high spec computer by buying a readymade.
Processor - don't waste money on 64 bit Quad Core - (only my opinion) there is little about that can use it.
Graphics and RAM already matched and adequate with computers from £400 to £700 - minimum 2Gb
Vista if you must!
The high volume firms automatically install it without an option.
They also install all sorts of software which you may not want (Works and Norton).
XP if you prefer a known and v.g. OS.
If you want XP (and I would) then this narrows down the field of suppliers. I know Novatech have this option, and there are others.
Overall the choice is huge and unless you have your mind already made up, stay with a ready made PC to suit your budget, rather than spend extra money to have someone devote a working day to your requirements.