I've touched on Vintage Computing. Twice, actually.
This is a good theory. Now how do we turn this theory into a literal set of instructions and ideas for getting and using computers? What do we need to think about?
1) Think about what your needs are.
If you need a Mac, buy a Mac. I'm serious. There's no point in buying a cheap old desktop for Linux when you need things from OSX, and trying to struggle through and make do with what you have.
However, if your needs are adaptable, then think about which distro you need, and make standards so that you know what you're aiming for.
Are you only comfortable with Ubuntu? Unless you're using an older version of Ubuntu (eg. before 8.04), you're probably going to need a more powerful computer.. Ubuntu can run on any processor past 2003 without issues, but you might want to make sure you can get your hands on at least 512 MB RAM before grabbing that old laptop off of the thrift store shelf.
On the other hand, if you're content with a Debian with Fluxbox or console, maybe you don't need quite that much RAM, and could get by with 256 MB RAM. Just think about what you use your computer for. Firefox? Flash? Video? Pictures? Music? These things require a processor that's a little bit newer than a 2002 Celeron. Maybe you could do it if you learned a bit more about the console. Are you willing?
Remember, while we're all about the re-use of old computers here, we're also about minimalism. There's no need to get a computer if you're not going to be able to use it fully.
2) Consider the situation you're in right now.
Remember that whenever you're comparing items to what you already have, the sutff you already have has a huge home-field advantage. You already know it well. It costs you nothing to keep using it. It has no environmental impact past the initial creation.
Are you looking for something that will make you more productive? How? In what ways? Quantify these allegations. Assume that your current position is the greatest computing platform known to mankind (because it is). How does this thing help you?
3) If you already have a computer, find a way to pass it on in a healthy way. If you can grab it and give it to family in need, go for it. Customize a Linux install and give it to a family member or school. throw it on Craigslist or even give it to a neighbourhood kid, give him an Ubuntu disk and tell him to explore it.
If you have old gadgets, get rid of those too. Apple takes old cell phones and iPods, and most old computers (sometimes giving you gift cards for the pleasure). Best Buy will recycle all old stuff bought from their store. And so on.
4) Hunt. But hunt with standards. Remember what you thought about in Step 1.