Well, if you're comfortable installing the OS from scratch, and you just want a computer to run most tasks, don't have any requirments, such as high-end gaming, burning to multiple burners, or using multiple hard drives, these usually run pretty cheap on eBay:
I've been eyeballing them for some time now, but haven't spent any money yet. I've watched a lot, though, and I've seen them go under $200 for a practically brand new computer.
They are cheap computers, but I've also seen 2 of them in action, personally. One with a Pentium Dual Core CPU, and one with an Athlon X2 processor.... or it may have been the X3. Either way, both machines can do pretty much anything, they're quiet, convenient, and tiny - so they fit practically anywhere. Also, tons of connections by default.
So, you can spend $160 on a refurb P4, or you could spend around $200 - a little more or a little less, just depends - on one of those Acers, and have pretty close to the latest stuff on the market. You just then need an Vista install disk for Acer (unless one comes with Windows 7 now, or maybe one is with XP - haven't seen in a little while on those)... if you don't have one or can't borrow one, you can likely find it online for cheap - if not direct from Acer.