I just did the exact mod on this desktop. Started complaining bitterly with 512K and Firefox and the Trend Micro anti-virus, spyware, and anti-spam suite.

Purchased another 512K from Crucible for about $60., took 30 minutes to blow all the dust out of the interior, took 30 seconds to plug the memory in, and now it's back to reasonably quick. Not like brand new.......but not objectionable.