wich 400 dollar range do you want us dollars or euro dollars?
I'm pretty sure it's still pounds in UK. And 400 pounds is about 800 dollars.
Heres what i suggest, these btw are newegg.com prices as I'm not familiar w/ the layout of other sites. For UK conversion divide by 2, hopefully it comes out accurately.
Intel C2D e4300($117)
any 7900gs(approximately $150-170)
MSI P6N platinum($145)
GSkill 2gb RAM cas latency 5($90)
Seagate Barracuda ES ST3320620NS 320GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM($95)
Thermaltake CL-P0114 120mm CPU Cooler($45)
Rosewill RP500-2 ATX 2.01 500W Power Supply($50)
Case-Cooler Master Centurion 5 w/ window($50)
This totals out to $752 which is quite expensive especially w/o an OS or a mouse, keyboard or monitor. If you down grade a few things like the ram and the gfx card and maybe the mobo. You can squeeze those things in. I'm not sure if youre gonna buy your OS or "get it". I also recommend waiting for the intel price drop on July 22nd which can save you some money.
EDIT- i may just have done my math wrong, its summer im not trying