I can give u an idea, I have a POS HP dv2415z, AMD 64x2 tl-56 with a gforce m6150, 7200 rpm sata, 2gb ram on vista ult. 64. its a crap machine, but just a get me by until I get a black 15x monte or a 15" voodoo envy etc
With 1 gig ram, a super clean install of vista, from cold boot, once os loaded and breathing easy I launched Firefox, 50 seconds to full load uncashed, closed it and reopened it, 20 seconds cached.
Same thing with 2gb ram, FF uncahed load 5 seconds, cached load 2-3 seconds!!!
So yeah, gettin above 1 gig makes a HUGE difference!!!!!
going to 3 or 4 just means less page file swapping because u'll have plenty of overhead to load into after vista, and larger and more programs can reside in ram for quicker direct loads and cached loads while having more room to cache many and much larger programs without going back to the hdd or page file swapping.