This is probably a daft question - but how do you know when you need more memory? I have 3GB in my 2009 MP (Quad 2.66) and istat usually says I'm using around 70%, but in activity monitor 'real memory' used totals to about 730MB. When I run top in terminal I get this: Also, will Snow Leopard change things? Will I need less memory when that's installed? Finally, I guess Kingston memory is ok? It says it has thermal sensors, and I'm thinking I might as well get the 6GB kit -and- the 2GB module as that works out only £10 more than getting just 6GB from crucial (who don't mention the thermal sensors either!).