Should be. Trainz is very CPU reliant so anything over 3ghz should be sweet. I have a i7 950 running at 3.2ghz and trainz runs pretty well. I also like to run trainz in a window so I can do other things while trainz is running, and my PC handles this quite well. (not sure about the video card. I am a Nvidia fan-boy. So I am not sure what a good ATI card is.)
My system hardly ever uses more than 30% of 8 gig of RAM so you should be ok with your 4 gig.
Check the card out on Toms hardware, see how it compares performance wise.
I apologize, but I thought Trainz was more of a Graphics hog? I have a 3.0 GHz AMD currently, and between switching a 240gt and 450gts, I can see major differences.
I have 8 Gigs of 1066 RAM and Trainz still occasionally stutters in high detail areas.
On the plus side my GPU (Palit Geforce GTX 460) is very reliable.
you having a 32 bit system isn't gonna make a hill of beans in trainz, windows 7 only uses about 1.5 gig at idle for me, and trainz wont really use more than a gig of ram during gameplay, you should of went a little higher on your graphics card though at least an ati 5770, as i have stated in other posts trainz is more cpu dependant that anything, fastest hardrive that you can afford helps alot as well, the ssd drives will make a big impact in the future as the hardrive is the bottleneck of your computer