Must be a pretty old PC if you haven't got a PCI-Express 16 slot... Even with the old AGP slot motherboards, you should check to see if it's AGP slot is, 2, 4 or 8. If you are going back to your era of motherboards, alot could only take up to 2 gigs of SDRAM !!! - (Trainz runs better if you have 3 or 4 gigs of Ram).
Wouldn't hurt to give us your PC spec's and then we will be able to better advise you as to what we think is your best option/s. - (as the old saying goes, you are probably better off keeping the bath water and throwing out the bath)...
In other words, you might be better off putting the video card money into upgrading your motherboard, CPU, ram, video card & power supply, & possibly case. Might be even simpler to just look at a whole new "gaming" PC...
Not knowing your budget or your exact PC spec's you could pick up something like this for $40 - $50 bucks; NVIDIA GeForce 6200 512mb AGP X 8 or a Matrox Mellennium P690 Plus LP - PCI video card 256 mb DDR2 for $100 - $150+
Cheers, Mac...