ram is always a help, but you don't need it for this problem. i'm pretty sure i know what you are experiencing. someone mentioned about 15% hd space should be clear and thats true, except that it will gradually drop below that. idk if its a virtual memory thing or what but after a few hours or few days depending how full you are you will notice it really slows up until you reboot and then its fine for awhile. and if you pay attention to your HD free space when you freshly boot up and look at it when you start getting that slowness or haha "slowlyness", you'll notice its free space has dropped by the gigs.
its time to go to town and delete delete delete, onyx won't clear up a significant amount... its only a small help. OR get a new HD. my addiction is movies, porn, music, and music videos. i added some new drives and store media seperately and put my os on a 110gb HD. being that it was a newer HD it has faster speeds and i don't worry about the "virtual memory" problem like i did when my os was on the 30gb. if you do this, get carbon copy cloner its free fast and fantastic!