An upgrade to an SSD on any SATA interface (I, II, III - 1.5, 3, or 6Gb/s) will be a dramatic improvement. Sequential and Random reads/writes are substantially higher on an SSD compared to any mechanical drive in any situation. SATA is backwards compatible, you could (and should) purchase a SATA III SSD, it will perform fine on your SATA I interface - plus - you'll be able to take advantage of the newer SSD chipsets and a more mature feature-set. I do suggest getting one based on a Sandforce chipset.
SSD's longevity - 4.5 years? Wrong. they last significantly longer than that and have an average lifespan about twice as long as traditional mechanical harddrives. This measure of lifespan is a huge misconception in current market SSD's.
My Western Digital RE4 mechanical drives have an MTBF (Mean Time Between Failure) of 1.2 million hours, my Patriot and Corsair SSD's all have 2 million hours with an enterprise class Corsair having 10 million hours. I have a 6 year old Samsung SSD that acts as a cache drive in a heavy use server, this really is a worst case situation for an SSD and I've had no problems at all and it's going on six years old.
Best of luck.