I have a single green cable linking the PC and subwoofer, as you said. Hopefully it will work out.

http://img442.imageshack.us/img442/6797/p1000799qd6.jpg

Will this work considering my speakers plug into a subwoofer?

What I have: http://www.amazon.co.uk/Altec-Lansing-MX5021/dp/B0000C2KB7/ref=sr_1_36?ie=UTF8&s=gateway&qid=1202564857&sr=8-36

Excellent for you, but I did not understand any of that.

My main concern is, once I plug everything in, what hardware do I need to switch between PC and xbox instead of unplugging/replugging the wires every time.

I'd like to buy an RF switch to exchange viewing and sound between my PC and xbox when want to, but I am incompetent when it comes to hardware.

I was told I needed to do is to plug the switch cable into the RF port on my graphics card in order to get both speaker sound and display. Does the NVIDIA GeForce 7800 GS have such a port?

This is alright, but not great: http://dinopc.com/shop/pc/viewPrd.asp?idcategory=29&idproduct=115

Would it be able to handle the said games at least to a playable extent?

Let's assume he is. He'd like something that would be able to run Crysis/Unreal Tournament 3 at medium graphics.

pcgamer_07, I'll keep that in mind.

A friend of mine is stuck with a terrible, out of date machine. His limit is 500£. Would anyone be kind enough to suggest the optimal pre-made PC he could purchase at his budget- that is, for gaming of course.