think of it this way, the pc memory is the storage area for the data for the game, the agp port is the motorway to the gfx card, the memory on the GFX card is the storage for the gfx and textures/shaders being used in the game, the 2x, 4x, 8x are the number of lanes leading to the card (like a motorway), the more lanes the faster the card can load data, but the card runs at a speed all of its own.
a cheap 8x agp card will run no faster than an old TNT2, a fast 8x card will blow your socks off, the only time the port slows down the card is if you need to swap huge amounts of data onto the card in a very small amount of time, and that doesn`t happen often, your stuttering before was not an issue with the port, just that the GFX card was well dated and struggling, the GFX card is a seperate computer dedicated just to rendering GFX, nothing else, the speed of that card is what gives you results, I know that on a lower spec athlon cpu (1.4ghz) with agp4x and a 128mb fx5200 graphics card, and 512mb of system ram, that Unreal tournament 2004 and Farcry run ok, as do RTCW and anything before that.
thats pretty good for older technology, from what I read of your specs your bigger issue is going to be the CPU, 1.5gighz will start to look slow in the next few years, any modern card (eg fx5200 ultra) should suit you fine.
