Here is some of my experience with upgrading Macs.
When I bought my MacBook Pro, the maximum supported memory was 4GB, so that's what I bought. My Mac worked fine, but sometimes I would have a lot of programs open, and it would get sluggish. Eventually I'd have to reboot. Recently, a colleague who has the same model of Mac as mine reported that he had read that an Apple firmware upgrade in 2011 had changed the maximum memory to 8GB, and that he had installed two 4GB memory modules, and his machine was working fine. I searched the web, and others reported the same result. In 2012 I bought an 8GB upgrade kit from Other World Computing (about $50), watched the video, and installed the new memory. Installation was easy: 8 little screws. My Mac works fine and is snappier, and so far, shows no problems. If the system misbehaves or needs service, I'll put the old memory back in.
The most recent Retina MacBook Pro and MacBook Air computers (2012) do not allow memory upgrades; the memory is soldered in.
I am tempted by the thought of replacing my Macbook Pro's hard drive with a Solid State Disk (SSD). People who have done this report that their machines are much faster. Disk prices have gone down a lot: one idea would be to remove the optical drive and replace it with a 750GB hard drive, as well as installing the SSD. There is some discussion of whether or not one should use TRIM with SSD drives. Read the OWC article http://blog.macsales.com/11051-to-trim-or-not-to-trim-owc-has-the-answer for an opinion. There are assertions that "TRIM on non-Apple SSDs conflicts with FileVault2. If you enable both, your drive will be corrupted."
Apple supports SSDs on some models of the MacBook Pro and Air, but you have to order it when you buy the computer.
Apple charges a lot for memory: some people buy the minimum from Apple and then buy third-party memory and swap it in, and save money. The downside is that if you have a problem with your Mac after you do this, and take it to an Apple dealer in for service, they sometimes return your machine unfixed, with the memory in a plastic bag, and a note saying that non-Apple parts were found, and they don't work on non-Apple stuff. In the past, there have been cases where third-party memory installed in a Mac worked fine, and then after an OS upgrade, the same Mac started crashing and acting up, and the problem was fixed by replacing the memory. This happened to me about 10 years ago, on a PowerMac tower, and the memory supplier replaced the memory. I think that now Apple doesn't consider replacing the memory on a MacBook as voiding the warranty.. but Apple Service probably wouldn't help with problems on a non-factory SSD.