A RAM upgrade is usually a fairly easy way to improve system performance, and is also pretty easy to do on most PCs.
I have a older Mac mini G4 that I use as my home media server - it pretty much just runs iTunes, SlimServer, and more recently, EyeTV. I don't use it for playing/ripping/encoding/etc so it has run "ok" with the stock 512MB of RAM included - performance wasn't great but it did the job.
Every now and then though, especially if I have multiple streams going around the house the machine would slow down, so I decided a RAM upgrade was needed.
I first checked out the Apple store. Unlike most PC's, the Mac mini is fairly well sealed up, so I'd rather have someone who's cracked it open before do it. Astoundingly, though, they wanted $400 to upgrade to 1GB. Considering the entire machine only cost $499 originally, I thought this was a bit steep.
Unfortunately, the mini only has 1 slot for RAM, so to go to 1GB, I have to purchase a 1GB stick of RAM and toss the existing 512MB. I found a few sites with good instructions for opening the case with a set of putty knives. This was a bit tricky at first but once open, replacing the RAM wasn't much more difficult than any other PC - just a lot less space to work with. The inside of the mini is as well designed as the outside - very packed in but tidy!
Getting the case snapped back together was also a bit tricky but I was able to complete the upgrade with only a scratch or two on the bottom of the mini.
Performance has increased quite a bit, not nearly as sluggish as it was previously. I still wish Apple had designed the case to make simple upgrades easier (similar to laptops) but overall it wasn't too bad.