The 10D is a great camera. I went from a 10D to a 1DmkII this past January, and while the differences there are huge, IQ is actually not that much different. The noise characteristics of the mkII are a bit better above ISO800 (the 10D is fine up to ISO400, 800 is pushing it, 1600 is for emergencies only, and 3200 is more noise than image, especially if the exposure is off even a bit).
But this isn't the comparison you're looking at, I suspect. The 10D is a fabulous camera at ISO100-400, and will give you fantastic 6MP images to work with. And it can be had for steal, as you say. $200 is a good price.
All that said, would I buy another 10D? Nope. The 20D really is a huge upgrade. Just the ability to take EF-S lenses is a massive step up (lets you get into really wide lenses, if you want to). The IQ jump to a 20D isn't huge, but you do get more MP and slightly better high ISO noise performance (in addition to EF-S lenses, as I mentioned). And a used 20D can be had for not a lot more.
Skip the 10D. It was a great camera, but you can get a lot more bang for your buck these days in a used 20D or 30D.