I have been installing Ubuntu on older PC's for a few years. I find there is a difference between what you need to install versus what you need for a satisfying user experience. For example, I installed Hardy Heron on an old Pentium III box, with 512 MB of RAM, and 64 MB onboard graphics. It seemed like it was strong enough but, While there wasn't any problems installing from the CD I burned from a downloaded ISO file, running it was a different story. It would frequently hang, especially with OPEN OFFICE word processor (30 page project) and Firefox browser open (I think had 4 tabs open) at the same time. Could this be an issue of not having enough RAM? It very well could be, but system monitor showed only about 75% of RAM being used at the time. I was working on a research paper at the time, and lost quite a bit of work because the only thing I could do was restart. So my point here is, you need to provide the hardware environment that Ubuntu will run well in, not just focus on what you need to install. I'm thinking that for Ubuntu 10 you need a Pentium 4 at LEAST 1.8GHz, with at least 1024MB of RAM and an Nvidia Geforce MX series at least. Also, please use a HD at least 40GB; while smaller drives will work, I would worry about running out of space, which from Windows experience can be devastating. Ive had better luck incidentally, with Intel System boards and chipsets than other makes.
I am all for finding ways to keep using older computers. I have never liked the idea of throwing something like a computer away simply because it's considered obsolete. I think there are a lot of users who really do need more power, but there are a lot of other people who only have basic needs which are easily met by an older computer, but they can't afford to buy a new system. Meanwhile, I've seen piles of old computers 12 feet high that will be scrapped, and there are people who don't have anything to use; I think it's an obscene example of waste.