Well, if you're using the 32bit version you're limited to around 3GB all up for everything to share, and there may be a (slow) swap file involved, but the 64 bit version can access as much as your PC can take, as well as humungous hard drives / SSDs. To answer your question though, I don't think so, I believe it uses what's available as needed.
Having mentioned drives... are you using SSDs? They make a mega-difference!