what everyone said..any 667 ddr2 will work..there are some REALLY good sellers on the bay that sell QUALITY ram (i have bought from them several times for a variety of machines)..funny how the prices of these things move in cycles..it is...
acer is a decent low end machine..they do sprinkle some higher quailty parts in with their packages but their customer service is a little spotty at times
i was running 64-bit myself and just found that many of the third party apps that i really loved and used on a daily basis just didn't play well so, i am out of vista totally and back to xp pro...happy as a pig in slop!