For something like Halo, realistically you will need a 128Mb graphics card, 1Gb Ram and something like an Athlon XP2800 as the minimum. Even with that, you will have to turn graphics settings down a bit (I ran Halo on this PC, which is these specs)
As far as "cheap" goes, you'll need to give us your definintion of "cheap"! This
http://www.aria.co.uk/Products/Syste...roductId=30032 doesn't seem too bad and will be fairly "futureproof" (you will need a monitor, but I guess you have one), but I have no idea if that is within your budget or not.
Oh, I've bought stuff from these guys before, they are reliable