Probably the best bet is to use an online computer manufacturer so that you can define the system that you want, with regards to parts, and the added bonus of an XP disc without all the garbage put on the system by the high street systems or Dell. This is wher I got my current system from
http://www.eclipsecomputers.com/. Here is an example page of how to define your system mix and match and it will price it for you.
http://www.eclipsecomputers.com/systems.aspx?sysid=3# most other retailers use a similar system