Before we look at how you can save money, you should know something about upgrades. For many people, some kinds of products seem more appealing because you can upgrade them somewhere down the line. This may work for old tools and kitchen appliances, but the computer industry doesn't think that way. They want you to buy something, use it for a year, throw it away, and then buy the newest model because they wouldn't make any money if they actually had to offer upgrades for the dinosaurs of the past. Really, computers just aren't designed like the Stanley #5 jack plane, which was offered in the same basic form for 115 years.
If you see something advertised as being upgradable to some future specification with some extra piece of hardware, it probably isn't going to do you a whiff of good, especially if that extra piece of hardware doesn't exist yet. It usually means that the company has a newer product that isn't quite ready, and therefore, the marketing department is trying to think of ways to milk an old product. If that little upgrade actually does appear, it will most likely be crippled because the old and new technologies are so different that the engineers had to do some really bogus stuff to get the things to talk to each other. I could bring up countless examples, many involving CPUs, but you should take my word on this one.
So what can you upgrade, and what kinds of parts can you carry from an old computer to a new one? Disks, for one, are always fairly easy to add to a machine, whether a disk is new or used. Memory is a little more complicated. For the most part, you can always add more memory to your existing system, but keep in mind that you may need to remove some of your old memory to make room for the new stuff.
You should regard the motherboard, processor, and memory as one unit when purchasing hardware. Don't even think of carrying just one of these components over to a newer system (well, this is a slight fib; sometimes you can keep your memory, but it's very rare). You may also need a new power supply, case, or graphics card when you upgrade other hardware on your system, because the standards for these components are not terribly stable.
Peripherals and USB devices are easier to carry from system to system. There really isn't any need to change peripheral devices (like printers) if what you have works. And if you have a keyboard or mouse that you like a lot on your old system, then in the name of your own sanity, don't get rid of it. I hate to say this, but they just don't build keyboards like they used to. It might be worth buying a special adapter to make an old keyboard compatible with a new system (for example, to convert an AT keyboard to USB).
Upgradable firmware (embedded software) in external hardware components (such as modems) is also a good thing, because it's much easier for a company to write software to support new protocols than to develop and produce hardware fixes.