It's a good idea to be frugal with your orchestra. Only use the instruments that you need .. rather than have them all sat around .. just because! I use a similar setup .. a Macbook Pro with 8 gig RAM. Typically I have no more than 30 instances of VI loaded, which gives me pretty much everything I need. Not to say I don't want more .. can never have enough instruments :P .. but if you're limited in CPU and RAM then you have to make choices over how much of what you use.
You can still get a huge sound with a smaller template, you just have to try to do more with a smaller number of instruments. It can actually help you to write having these limitations because you have no choice but to do more with them. Rather than just layering other stuff on top.