Any of the middle to top end graphics cards should do for Trainz. Having said that, it also depends on how much money you want to spend on money and also on other components in your existing PC. Whether you have a (very) fast CPU, fast memory, USB2 or USB3 hard drives etc. in your PC. The warning regarding having a large power supply is a valid one, any power supplies from, say, 700 Watts onward should be OK, the larger it is, the better, as modern cards now usually use 2 separate power input cables because they do need it.
The other warnings regards better graphic cards being larger (longer) apply too, hence having a suitable mid size tower PC case to accommodate such longer graphic cards is mandatory. These cards will not fit into the common mini sized PC cases anymore.
You did not give us any specs of your existing PC, as by just upgrading a graphic card may not solve stutter and slow operating of Trainz. As said, the other components in your PC will have to be reasonable matching too for good performance.
Price range on graphic cards depend on where you live and from whom you want to buy one, do a Google search for graphic cards pricing in your country or check EBay too.
Lennard
