Tuning your VM should be based more than 1.5 x memory. I/O on disks is SLOW and Windows loves to swap out to disk. If you are running a low speed hard (i.e. 4500rpm) drive, upping your VM could actually impede performance drastically. Personally I keep my VM at 512mb, but I'm running 2Gig of high performance memory.
So basically play with it a bit, but I would suggest going no lower than 512mb for VM and no higher than 1.5 x your total memory, as a rule of thumb.
My 2cp.