For an "old" system, if it has 1GB (XP, 1.5GB or more for Vista/7) or more of memory, a disk upgrade is the single best upgrade.
I had a work laptop hand-me-down. Dothan 1.83GHz, 1GB of RAM, absolutely wonderful 1400x1050 screen and a woeful 4200rpm HDD. With the work SOE image, this thing would take 10~11 minutes to boot to the desktop. Swapped that Seagate out for modern WD BEVS 5400 rpm drive, boot time was 3.5 minutes. Not great but far more useable. Newer drives have far better transfer rates than older drives and are larger, so more of your data will fit on the faster outer zones of the drive. Just like interest, compound performance improvements.
Why mention this in a thread about SSDs? It's more about if you want to/need to keep an older but still useable machine around. A modern mechanical HDD upgrade might do the trick if the budget won't stretch to an SSD. I'm typing this on an Inspiron 6400 T7200 that started out with a 5400 rpm WD. It's now got 4GB instead of 1GB, and a 7200rpm 500GB Hitachi, and you can definitely notice the speed improvement. An SSD is the next upgrade. Or a Llano based 13.3" 14" notebook, because I really want more graphics oomph, not a lot, but more than Intel 945G, plus decent battery life.
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