Any 32-bit application can use 2GB (or 3GB) of RAM as its maximum. This is 32-bit world limit. The 64-bit applications can use much much more. You can reach the limit easily if you use large sound linraries (like Symphonic sounds). Also some plug-ins (for example Referbs) can be pretty RAM consuming.
You can use 32-bit plug-ins in 64-bit Cubase. There is build in VST Bridge. Of course, as every bridge, it takes some CPU power extra. There is also 3rd party bridge, which works with some plug-ins, VST Bridge doesn’t work with: jBridge. Again, it takes some CPU power, but then you can use more then 3GB of RAM for your project and 64-bit plug-ins.