For 250-300.. knowing ur prices, i'll suggest the optimal hardware set.
Since u already have a case and u bought a new power supply, u need a new motherboard, CPU and RAM (video card is optional, since the motherboards ill suggest have quite powerful integrated cards.. which are stronger than ur 7600GS)
MotherBoard: Gigabyte models (any kind) with AM2+ or AM2 processor sockets (AM2+ supports AMD Phenoms/Athlons, AM2 - Athlons/Semprons)
or ECS Elitegroup 8200 (AM2+ socket) - along with an integrated video card and full SLI and Dx10 support.
All of them have PCI-E 2.0 video card slots and 2-4 DDR2 and DDR3 RAM slots.
Prices - 30-60 GBP
CPU: AMD Athlon 64x2 5200+ Dual Core or better. (AM2+ sockets can support AMD Phenom Quad cores)
Prices: 25 and up GBP
RAM: DDR2 2x1, or 2x2GB 800mhz (more wont fit in 250-300 GBP limit). And it is important to get 2 equal ram sticks, for Dual Channel system to work, which boosts up the memory speed quite a lot.
Prices: 15 and up for each stick (depends on the manufacturer)
Video Card: GeForce 7600GT - 8800GT (higher wont fit in the 250-300 GBP limit) and those are the most stable cards of them all, they are safe, they dont burn or break, unlike the 9x models.
Prices: 50 and up GBP
HDD: You should test ur current one, if it works, you dont need to change anything, if not, get any1 u like, which has at least 7200 RPM speed and has SATA connectors.
Prices: Differs from the speed, capacity and manufacturer (Preferred - Samsung HDD's).
The rest is up to you, but this is already a tested set, which works very well and with very good performance, and will run any game up to date (if u'll get GF 8600/8800GT, will run Crysis/STALKER Clear Sky on max too) ^^
Some others may suggest Intel CPU's or ATI video cards.. trust me, thats not a good choice, especially if you want a good gaming PC.