I have found that Vista is in itself a memory
hog. Forgive me for this and I know someone will be able to be more acurate but XP used to be able to run fully in around 750 Mb of Ram.
Vistas total is much higher and it has alot to do with aero. If you are using Premium or Ultra with the aero interface the Ram usage is jacked up considerably higher than the more basic versions of Vista without the aero interface.
I run 2 Gb of DDR2 and my Ram usage is higher
than I would like, saying that game performance has not really suffered from my point of view.
Memory is pretty cheap now and a couple of Gigabytes of good quality DDR2 can be had for just over £110 for 2 x 1Gb modules. I would fill up with as much as you can afford.