I have not heard of a single VS instance taking over 4GB of RAM. The only time you'll get into trouble is with multiple VS instances - and there 40 bucks for another 4GB of RAM pay off instantly.
While VS itself doesn't consume large amount of memory, Resharper is a voracious memory eater. I used to regularly encountered OOM exceptions - not due to insufficient memory, rather because of excessive .net memory fragmentation brought about the holy marriage of VS2010 & R#. IIRC, this type of memory problem did not occur in VS2008 too frequently. Recent versions of R# have ameliorated the problem to a large extent however.
