Just make sure you know what you are doing and the main things to look at are:
Motherboard, Hardrive, GFX Card, Ram And Quad Processor.

Hardrive:160-250gb

GFX card: I personally like the Nvidea's depending on what your PC works best with.

And Ram get 4gb if you can 3gb is still exceptional.

1) Make sure your parts are compatible
2) Get quad core if possible
3) The best bang for buck GFX card IMHO is the nVidia 8800GTS, I would highly reccomend it for a budget system.

Good luck with it

like stated make sure parts are compatible
id personally go for an intel processor and an nvidia gfx card
when buying a hdd if you want masses of storage make sure you get a smaller hdd for your os so it doesnt increase boot time, i found booting off a 1tb to be slow and laggy.

Dont compromise for a cheap mobo, cheap mobos may look the same as there more expensive friends but performance is compromised, a cheap mobo will have a shitty north/southbridge meaning slower communication between ram cpu etc so sticking an expensive cpu in a shit mobo may have its downsides.

When buying a motherboard and a processor rememebr to check the motherboard has the correct socket for the processor eg. socket lga755 is most intel processors pre dating i7 (excluding the old steam powered processors ofc).

I've just bought a new pc for around £600

It ain't current top of the range specs but still uber fast

500 gb sata HDD
4 gig ram
1 gig Nvidia graphics card
2.6 Quad core Intel Processor
Biostar Mobo
DVD Dual RW
Firewire
24" Acer Widescreen
XP Pro (32bit)

Be careful tho as I've found out that my processor is only for 32 bit so upgrades to 64 not possible without total rebuild

You get what ya pay for

Sno

I have some parts for sale like ram hard drives dvd drives mother boards graphics card and a few other stuff fans and shizz