A lot of people seem to stick with their old hardware because they can't afford new hardware, or they think it's too expensive to buy / build a new computer.
Let's say I have a system with a Pentium 3 Processor that requires pc133 memory required by that architecture. Most likely, the system will have very little memory, especially by today's standards. If you go to a site like Newegg, $50 may at first look like a bargain for 512MB of memory. That is, until you look at prices of newer types of ram and realize you can get 2GB of DDR2 for the same price. It's almost impossible to find 1GB modules of memory for these older systems, as that was unheard of at the time. Even if you were able to find them, most motherboards at the time didn't support that large of memory modules. Then you start to look around at prices of everything else, and realize that computer components are dirt cheap these days. You can get a dual core AMD processor that will completely smoke your ol' P3 for a mere $56. Of course you'll have to get a motherboard and memory that will go with the processor, but you can get a Foxconn or similar motherboard with everything (sound, video, NIC) integrated for around $50.
Most likeley, you're waiting around a lot with your current system, especially if you're trying to run a more modern OS than Windows 98. Trying to upgrade your old system just isn't worth the price and hassle, for the amount of processing power you're going to end up with. New components are typically much more power efficient than old components as well, and will help reduce your carbon footprint and your electricity bill.
Next on the agenda is CRT monitors. These beasts were big, bulky, and consumed a large amount of power. If you're still using one of these, you could probably cut your electric bill by at least $25/month just by upgrading to a more power efficient LCD. LCD's are just as good or better quality than CRT's, and they're easier on your eyes. Refresh rates of 60hz on a CRT would cause a lot of eye strain, but that's a thing of the past with LCD's unless you have the brightness set way too high.
Another aspect that many may not think of is virtualization. I have a system that's powerful enough to run my OS, plus any other virtualized os's I may want or need to run at the time. This also saves me money, since I don't have to have extra test servers running, sucking up electricity and heating up the room. I've tested all sorts of OS's this way including Fedora, Ubuntu, Windows Vista, PCBSD, and any other beta release I feel like testing at the time. It saves me a lot of time, trouble, and hardware to just load them up as VM's, instead of using physical servers. Virtualization on an old P3 just wouldn't be that feasible.
So really, the best thing to do with those old systems is to donate them to a charity or school, and fork out the cash for some shiny new hardware. You'll appreciate that you did once you start using it and see all the advantages.