When I do a "File > Purge memory", it works if there is any unneeded memory available (with a progress meter and a new larger "Resources" number) - otherwise, nothing happens (no messages). 

I dunno exactly what memory gets purged and when memory becomes "purgable". Prolly info in the manual.

My system specs are identical to yours, except an Intel Core 2 Quad Q9300 on an Intel mobo.