I think that if you're trying to get more legs out of an older system, a SSD is definitely a great way to do that. Especially with laptops from the past four years -- a Core 2 Duo processor in a laptop still isn't fast, but a decent SSD will make it feel like a new system (at least in my experience) and then some. I think trying to make your Pentium 4 system better with some solid state storage is a lost cause however.
[citation][nom]MAGPC[/nom]SSD is just too exaggerated, yes it is faster than HDD and may be more reliable but that doesn't justify its price.Waiting until it is price gets reasonable.[/citation]
How much is reasonable? A 64GB Crucial M4 is $105... that's pretty damn reasonable to me. For that kind of money you could get a low-end mobo, an Athlon X4, or 16GB of DDR3. Upgrades don't get much more reasonable than that. But if you already have a decent, if older system, installing an SSD will make it feel like a brand new system should for the least amount of money.