jorga, one of my computers only has 512MB of physical RAM but is running 12 MT4 instances fine. my other computers have way more RAM. Usually MT4 doesn't need much RAM*, but it is some kind of windows resources (my guess is window handles) that there seems to be some stupidly low finite supply of (since I had the same problem before on other computers without even MT4 installed - but lots of apps running)

so I doubt 64bit windows will help much.

* I guess some experts and indicators might eat up way more RAM than others.