You can’t use more than 4GB Ram with Cubase 32Bit alone. (and VST plugins)
if you use 64Bit Cubase you cant use as more than 64 instances of 32Bit-Vst-Pluins, (because of a bug in Cubase) without jBridge. If you don’t need more than 3GB RAM you can use 32Bit verson of Cubase if you need more because you use many Plugins you might get problems because Cubase is running out of RAM and might crash. I use the 64Bit Version of Cubase with Automap, jBridge and TabSel’s Plugin Manager. Many people say it might be better to use 32Bit cubase on x64 windows because “better performance” and jBridge can also use 4gb Ram per Plugin instance if needed… but i would recomand C6x64 with jBridge because 32 Bit is past and if you use many Halion for example its better Cubase has more than 2-4GB.
You have allready tried to incrase the asio buffer?