Hi there
I run Railworks 2 on a 13" Macbook which has the previous Nvidia card in it, the 9400MG. I only have the standard 2gb ram too.
Railworks runs great on the Macbook, I have had hardly any problems with it at all and I think given you have the 320M in yours and additional RAM it should run even better for you.
All I will say is make sure you update the graphics drivers from Nvidias site as the ones included with your OSX disc will be out of date in bootcamp.
The only time Railworks suffers, or any game for that matter that I have run is when you turn on Anti Alising. That complete eats power and the game will slow down, but given the 13" screen and running at 1280x800 turning on AA hardly makes much of a difference, especially if you are like me and grew up playing Mario on the SNES and N64 :P What are a few little jaggies!!
If you play around with the settings you will find your optimum for running RW. Even when the settings are on medium it still looks fantastic and since the last update where they shifted some of the rendering onto the GPU as opposed to the CPU it runs better too.
The WCML route is a little taxing on the Macbook so when running a route like that I find the settings on medium work best, then you don't get any stutters.
Go for it, I think you will be surprised. I have been amazed just how many new games the Macbook is capable of running under bootcamp!!