Depends on what you would do to utilize those 4 cores and 4GB of RAM.
If you're not running 64-bit OS you'll see somewhere between 3GB and 3.5GB of available RAM for your 32-bit OS. If you don't do a lot of multitasking and would actually utilize 4 cores I personally think they're more of a gimmick or as some say an "e-penis extension". There are some games and some programs that can use 4 cores, and no I don't mean the multi-core compatable Prime95 stress either...but really I think quad cores are over-touted right now....kind of like DX10 gaming cards were when the 8800's and 2900's first came out...and even up to now, not a whole lot of DX10 gaming out there, yet a lot compared to a year ago.
I'd say it would depend on how badly you want those upgrades, personally I'd stick with that 6600 and overclock it a little bit if I wanted some more performance. I am running 4GB of memory in Vista64, and I gotta say it's not too bad at all, I have yet to run out of memory, and things are running great...but in most instances I don't think I would've had 4GB if I didn't snag the deal on the second 2x1gb kit that I did.
Hope that helps!