There is a common misconception that computers slow down over time. This is just not the case. There is no reason for your computer to run slower today than the day you bought it. There is usually something which causes your system to become slow. Things like low hard drive space and perhaps applications running in the background that you weren't aware of can impact speed. A failing hard drive can also cause problems.
As you currently have a 5400RPM 120GB hard drive, you will see a significant change in the amount of time it takes applications to launch with an SSD. Keep in mind, an SSD is only replacing your hard drive, so it is only increasing the speed of loading data from your drive. It won't speed up your applications (except, again, the drive access time for applications that may rely heavily on disk use). As you only have 2GB of RAM, your system likely pages data to your hard drive fairly often. This is when your RAM is "full" and it dumps some of the data to your hard drive so it can load more into the RAM. When you have a low amount of RAM to start with, this happens more often. As an SSD is somewhat faster, this becomes less noticeable.
Instructions for replacing your drive may be found in your user's manual. You should avoid going with SATA 6GB (aka SATA III). While they will work, the SATA speed on your system is SATA 1.5GB. Even if you get a SATA 6GB drive, it will only run at 1.5GB... so there's no sense in paying for the extra speed. You probably won't be able to find a SATA 1.5GB drive... but a SATA 3.0GB (SATA II) drive will be less expensive than a 6GB drive and it will perform just as well on your system. The drive in your system now doesn't come close to saturating the bandwidth of your SATA 1.5GB controller. So, even with the older SATA 1.5GB controller, you will still see a significant boost in speed with an SSD.
Having said all that... basically, yes... an SSD will make your system noticibly faster, but it will not boost processing speeds for your applications. 🙂
