You should still have close to 3GB of available RAM as far as I remember (4GB limit at 32bit memory allocation tables and so on, - 1GB for the GPU and some smaller ammounts of memory in other parts), and with XP and a 9800GT that should not become the bottleneck.. though I am only vaguely remembering any of that, as I switched to Vista 64bit many years ago and Im currently running 7-64bit and far more RAM that I can effectively use. Only 2GB worth of RAM should still be enough for SSX when youre running XP anyway according to the sys-req on the product page so you should still be in the clear!
Also, if the new GFX card is a considerable upgrade, it would probably add more to your computer than the loss of available system memory would affect it, as it handles data more effectively and uses its 1GB better than the system would use it in games.
As for other games, play them with Task Manager running, alt-tab out of the game and into TM at a peak point in the game, and you should see how much memory the game is using. If your computer is still stuck with DDR(1) ram-chips like mine was a year and a half ago, upgrading could become a costly experience, seeing as to have more ram, I needed a new motherboard, but new motherboards couldnt use my old ram, so I had to get everything new, and then I needed an new processor as well, and by then I needed a new power supply. It might be better to turn down some settings just a notch or two and get the game running smoothly again..
