Welcome to the forums, Andy and to Trainz.
You'll never regret getting the program. I used to run TS12 on a GTS-210 so even your video card should handle the program fine. You can always upgrade your video card later should you need to - just check that you don't need to replace your power supply.
I will echo what the others have said here. Get the program directly from Simulator Central (N3V's shopping site), or one of their authorized dealers such as Jointed Rail who I highly recommend. Steam has its own issues including their DRM which affects the installation of add-ons and other 3rd party goodies, as well as patches and service packs. The Just Trains version requires you keep your DVD in the drive at all times for DRM issues. With either the direct download or purchase from N3V or Jointed Rail, for example you can be rest assured that none of these "extras" are included.
I agree with you on the cost of the real model railroad. Virtual layouts aren't quite the same, but they sure are fun and very much addicting as the real thing. You'll find that once you get the program it will consume your life as you build routes and experiment. The latter point is one of the best features, besides being able to build a 100-plus mile route without needing a barn. If you don't like how a junction looks, or a town looks, rip it out and do it again. You can go as far as import Digital Elevation Maps created with a 3rd-party program called TransDEM. Using files off of the major map sources, such as the National Geological Survey servers, you can build nearly any route, past or present with the tools.
In addition to the ease of use, truly there's nothing like building a real layout only to have to tear up the layout after spending all that money. Believe me. I've done that more than once in my life. Using a virtual railroad in this respect is the best of both worlds because you can move it from computer to computer and continue working on it since it takes up no physical space other than what's on your hard drive.
John