It is very effective if you have less than 2GB (XP) or 4GB (Vista) of RAM.
Also, the performance increase is very substantial on laptops, since most of them typically use 4200RPM and 5400RPM hard drives instead of the much faster 7200RPM desktop drives, and because laptop RAM is more expensive than its desktop counterpart.
And please be patient instead of multiple-posting; you are getting voluntary help here. This is not a paid-for tech support forum.