Glad to help
Having a full hard drive should have no effect on the performance of your computer, though. If things are running slow, it's because too many programs are running, or they're using too many resources. You might want to try disabling services you don't use, uninstalling programs you don't use, backing up those mp3s or whatever before worrying about installing a new hard drive.
As for installing programs to a new disk - you can devote the entire drive to /usr, for example, and modify your /etc/fstab to use the new drive's partition instead of your existing /usr partition. Of course, you should copy all the existing stuff you have in /usr over to the new drive. You could also use the new drive for some other portion of your filesystem hierarchy, and copy old stuff off your other drive to make more room on it. There are lots of ways to do it; it all kind of depends on what's using up all the space!