when you upgrade to a 64 bit operating system install both 32 and 64 bit sonar and continue using the 32 bit vsti (synths) in 32 bit sonar.

I wouldn't call a 50%+ increase in memory insignificant.

the only way you can get a definitive answer is by knowing what your needs/expectations are of Sonar.

If you're just using mainly audio with a few RAM-efficient soft synths then I'd say leave well alone and carry on as you are.

BUT, at the other extreme, if you plan on doing full orchestral mockups with something like EWQLSO then you're going to need a LOT of RAM and this can only be achieved in a 64 bit environment.