What's your resolution? If it's 1900x, then you probably won't be able to run Crysis and DA on high quality. Also, why do you need a new computer if your current one is running the new games just fine.
You proably don't need a PSU. What one do you already have?
I'm not in the UK, so I can't give you an exact build, but I can give you what to look for.
First, this may be over budget, but give it a shot:
Mobo: Asus P7P55D-E Pro
RAM: Cheapest 2x2 GB 1333 mhz CAS Latency 7 kit you can find.
GPU: HD 5770
PSU: OCZ makes a cheap unit, but it's still good quality. You'll need at least 450W. Also check the prices for Corsiar, Seasonic, Antec Earthwatts and FSP. These are all quality units, but prices may vary.
Second, this one should be under budget:
CPU: Phenom II X4 955
Mobo: Asus M4A79XTD EVO
RAM: same as above
GPU: HD 5770 (or if it fits, the HD 5850)
PSU: same as above
EVGA P55 LE 123-LF-E653-KR. It should be a little cheaper.
What's in the old PC? It might be easier/cheaper to buy some replacement parts than essentially building a new one. I would also find some way to test what's failed. So I would borrow somebody's computer (with the same socket for the CPU) for a day or so and do some testing.
Its got NF7, Mobile Athlon @ 200x12, 1.5gb DDR @ 200, 2600XT 256mb agp.
It went when i had to switch off at the mains after a crash. It will boot althe way to the login screen but there is no power to the mouse or keyboard. Thats why i suspect the power supply or main board. ive ordered a new (350w) psu to test it.
Any idea if ill be able to run XP on this new system i HATE vista?