Do you think it's coincidence? You could pull the new RAM out and see if the problem goes away, in which case you might want to check that the RAM has the same specs as the original RAM.
If it's coincidence, maybe it's time to throw away old temp files, then defrag the hard drive if you don't do that routinely.
Installed new antivirus software, such as Norton?
Are you using the default Microsoft player to play your AVIs, or are you using something like Videolan?
Have you upgraded to Vista, if so could you post the performance numbers listed in Control Panel > Performance information & tools > Windows Experience ?