well it depends...
see with windows 32 bit applications there is a set allocation of ram. meaning, 4 gigabytes is all you can utilize. no joke. this is fact.
so this is why 64 operating system bit kicks ass, to a point.... technology hasnt caught up with itself yet. windows 64 bit applications have a virtually topless limit and since you can run 32 bit applications with a 64 bit operating system, you can utilize your resources to your best..... but theres another catch, you can only do this if your DAW software of choice is 64 bit as well. even then, the variety and price of 64 bit plugins.... sucks.
but theres still an advantage of running 64 bit operating system with 32 bit software. with windows 32 bit, theres a cap of 4gb of ram. But roughly .6 gb is reserved for windows to run and function. so youre essentially only using 3.4 gb on any windows 32 bit platform. no matter how much ram you have, you cant use any more than 4, period.
but with 64 bit, windows 7 specifically, reserves 1-1.2gb of system processing. so, this adds the .6 gb back to the 4 gb limit with 32gb applications allowing you to utilize a full 4 gb if you have more on you MOBO.
plus if you have rewire devices you can certainly save or rather, ADD processing power. so realistically you wont really need more than 5-6 gb until 64 bit plugins are developed more.
now your chip is probably the problem here though. i upgraded last month and built a super computer. windows 7 64 bit, AMD 6 PHYSICAL (more than intel) core 3.0gb overclocked chip easily to 4 gb a piece, 8 gb of ram, 1 gb ddr5 pny grahpics card as well as other goodies....yeah its silly but it doesnt lag on a godamn thing.
lol
so my recommendations?.. add 1 gb of ram for $30, buy a quad core that fits your mother board, and definitely get 64 bit operating system and youll be in the same boat as everyone out there.....
EDIT: oh and dude, get rid of vista.
or you could pay way too much money on a UAD card or similar.