I'm not seeing that on a 1GB 2GHz P4 system (114MB consumed at the moment).
Of course, you are running an awful lot of tabs, whilst I'm running only nine at the moment and probably expect to possibly double it throughout the rest of the day. I can say with certainty though, that 3.5 was nowhere near as good, though.
I suspect you've possibly got a very javascript heavy site somewhere that's dragging things down.
PK