Hey, if you really want to save some cash, I've got a D60 I'm getting ready to put on ebay. It's a perfectly capable camera, depending on what you want to do with it. It does have limitations when compared to the 10D or 20D, but it takes perfectly damn nice pics.
You can save a bunch of money by staying a step or two behind the latest technology. The latest 'n greatest always comes at a bit of a premium, and unless you can articulate a good reason why only the latest thing will fill your needs, a step back might serve you just as well. Personally, I just took advantage of the recent dip in the price (and rebate) of the 20D, since there's likely a replacement on the horizon. Most speculation is that it will be a 1.6 crop, 2.5 LCD, 10-11 megapixel (to compete with the D200, but not the 5D), and prolly a better AF system, and priced at $1500 to $1600. Not enough of a jump up to make the price difference worth it to me.
If I remember the thread correctly, the reason Andy McFarland upgraded out of the D60 to the 10D was because the shutter in his D60 finally broke, and it wasn't economically viable to repair the D60. I believe he bought a used 10D and certainly seems to do quite well with it. A lot of us wish we could shoot swimwear half as well.
Anyway, my point is that there's no reason not to save a little cash and use dated equipment. For me, I shoot weddings and the D60 struggles to focus in low light. It's TTL flash metering is kind of tricky as well. In fact, I'll come right out and say it's not a good camera for wedding and low light event work. So I spent some $$ on the 20D to get the better AF and ETTL II flash metering.
I'm rambling. To answer your original question: As far as I know the major differences between the 10D and 20D is pixel count, a finer LCD screen, faster CF write speeds, a tweak to the AF system, and ETTL-II. Depending on what you're shooting, it may not be a big enough difference to justify the money.
Have fun shopping!