I don't think there is a diffinitive rule about what part to upgrade when it comes to this subject. I mean it is all dependant on what system someone is asking to upgrade, and more specifically what is their budget because £150 will not buy you a good enough processor for nowadays work, but it will buy you another Gb of RAM that will add huge amounts of speed to your computer.
By the way RedUK, you only went for a DVD-ROM, and not a DVD-RW?? Just curious as to why?
<center>Athlon 64 X2 Dual Core 4200+ 512KB l Gigabyte GA-K8N-XP Sli Motherboard l XFX GeForce 7800 GTX 256Mb (450/1250) l OCZ 2Gb (2 x 1Gb) Dual Channel Platinum Series DDR400 l Maxtor Diamondmax 250Gb SATA 7200rpm 16Mb Cache l Enermax 600W Noisetaker l NEC ND-3540A Black Dual Layer DVD+/-RW l Sony 3.5\" FDD l Black Coolermaster Aerogate 2 Fan Controller l Extreme Gaming Spider Case l Viewsonic VX924 19\" TFT 4ms Response Time l Logitek MX518 l Saitek Eclipse l Windows XP Professional Service Pack 2</center>