Kursah's recommendation to refund it and build piece by piece is sound advice. You can get an ATX case, good PSU, good mobo, chip, monitor, HD and CD drive for just as much as that system cost, or at least it would be competitive.
Chip Cost: 130
Notes:C2D 6600
ATX Case Cost: 30+
Notes: Any you prefer, as long as it's ATX or larger and has nice air flow
PSU Cost: 70+
Notes:I highly recommend the one I am using but there are others
Mobo Cost:80+
Notes: Try something like mine or an equivilant in the same tier. It's not EXTREME, but it'll be more than your brother will know what to do with.
Monitor Cost:130+
Notes:Any TFT 17" or higher with no more than 12ms(that's a spec you can look up on certain monitors; won't explain it right now)
HDD Cost:50+ ??
Notes:Not sure what they go for now. (Head size is important on HDD, but that's for another discussion)
CD Drive Cost: ??
Notes: Cheap, that's all up to you.
That's a terrible shopping list, but it points out that minus some cables, and possibly a few case fans, that it's more cheap than some Compaq or Packardbell AMD 3500 equivilant with a terrible board in it without any OC capabilties and stuffed into a Micro ATX.
The only benefit to buying a prepackaged computer..and I know this sounds absolutley shady... but you get a spare case, a backup PSU, a monitor, a spare chip, keyboard/mouse, some heat sinks and fans, misc nuts and screws and if you're DVD/CD or HD fail (which they do often on those machines) you can just throw them the product and serial number and get yourself a replacement
No, it's not worth spending 600 quid to get just that, but if you've already bought one you cannot return....
Cheers mate, look forward to seeing how you get on with this.