slow frame rate

i gotta ?
i bought rollercoaster tycoon3 and it had really crappy frame rate do to the fact that it was a 64mb integrated video card...so i bought a 128mb vc
and it runs a heck of alot smoother and improved graphics..but when the people start coming into the park the frame rate slows down really quick..to about 4fps i tried to bring the settings down a bit but it helps only a little. fs9 works great with the settings all the way up. qnd myst 4 and ms train sim...im new to the computer gaming thing but i guess all im asking is do i need to get a 256 or a 512...or do something to the computer without the use of base ball bats and hammers? money isnt really an issue...but i dont want to blow my wad either..
any suggestions.