if you are able to get say £500 as a sum you start to be able to get a very reasonable spec PC that will last sometime, instead of getting a rig that because you can't spend the extra £150 is more frustrating than anything else.
For £500, off of Scan, my younger brudda managed:
CoolerMaster Case (don't remember the model, but its quite swish)
Corsair 520W PSU
Asus M32N SLi Deluxe Motherboard
Athlon X2 6000+ CPU
2GB Corsair XMS2 RAM 800MHz
XFX 8800GTS (£100 quid barely used off one of his clan mates, I admit)
Pioneer DVD Rw
Which I [strike]helped him build[/strike] built, and it runs CoD4 reaaally nicely. He also got a decent Samsung screen out of Tesco which looks very spritely indeed.
I didn't think you could build PCs that spec for so little money, but I was surprised to find out that ... you can!
Also, our last family PC was out of PC world, and although it was fairly reliable (for a PC World package PC) it transpires that it was probably worth less than half of the grand we paid for it. GeForce 4 MX420 for crying out loud