512 should be enough for most general computing. Depending on your Operating system, finding out what is using up all of your memory might be easy. On win2000 and winxp, there is the Task manager. Under the processes tab you'll see a bunch of information, most importantly in your case is the Mem Usage column. If there isn't a mem usage colum, go to VIEW - SELECT COLUMNS - then select MEMORY USAGE.
By clicking on the mem usage colum it will sort all running processes by how much ram they are using. If you can't figure out what specific program goes with the Process name it, or post it here. My bigest hogs are actually microsoft programs. Explorer.exe takes 23,960k (23.9 megs), and iexplore.exe takes up 23,700k (23.7 megs)
Viruses can take up lots of memory and cause similar problems... poorly written device drivers and programs can also cause this type of problem. If you don't see the Mem Usage very high for any one process, it's likely a device driver... especially if the problem only starts happening after a few hours of use
