I had a computer that was about 5 years old (Intel E6300, 2 GB RAM, Win 7 x64) running business applications. I added an Intel SSD 320 series 120 GB drive. This made things feel a little quicker but was not a night and day difference. I recently upgraded the computer to (Xeon E31225, 8 GB RAM, Win 7 x64) and moved over the SSD and everything is lighting fast.
The computer is a system, which is only as fast as it's weakest link. In general if you access a bunch of small files, you will see an improvement, if you access a few large files you may not see much improvement.