Just joining in briefly.
512 RAM is IMO very borderline indeed ...... even if your paging file is generous.
YouTube and myspace type sites seem to be very heavy coding and are typical of examples I have noticed can bog me down too, on IE and FF. That said my RAM is quite generous but even so - I reckon the problems may only be greater if memory is not adequate and increase the chances of this happening.
Forum sites do overall seem very undemanding on resources.
Just noticed - Carbon has a point there re other resource-intensive progs.