If the easiest way to update an aging computer is by adding a solid state drive, then the easiest way to update a modern computer is to add two solid state drives in a low cost RAID array. The logic is quite easy to follow since your storage subsystem, even with a gang of SSDs in RAID, is still the slowest part of your computer. Of course, you are not going to see big frames per second improvements in games or gain the ability to calculate PI any faster, but what you are gaining is more important than either of those things.
With SSDs you gain instant access to everything, from Explorer to Photoshop. Gone are the days of the hourglass and all of the annoying pauses that by themselves are irritating, but when the time is calculated and a sum is figured you may gain an hour a day. Everyone always says that they need more time in the day - here is your chance to get it!
OK, I will end the rants and step down for a minute. Corsair has the Force 90GB drives at Newegg now for 189.99. The single drive setup is very good on its own, but a pair of drives is kind of like getting an extra gear in an F1 car. Take out the 7 speed and replace it with an 8 speed plus a KERS system that gives the RAID 0 array a little more boost when you need it.
In most tests today we observed two drives more than doubling the single drive performance. This phenomenon won't happen in every task, but some tasks, like loading game levels, it will. Running a dual drive, RAID 0 system isn't for the frugal. Even those people should be looking at a single SSD since you gain more in the long term than you spend.
If you don't already have a single SSD in your system now, I suggest you take off the blindfold and step out of 1992 (the year Seagate released the first 7,200 RPM HDD). Platter drives are based on very old technology and are no longer desired to boot your computer from. Use that platter drive to store long term data on.
I guess I climbed back on the box again...