I read that if you have Windows Vista, that you will not gain alot, for the $120.00 that you pay to re-install Windows 7 on an old PC.
If you have windows XP, an upgrade to windows 7 will do a complete OS re-install, and putting the old programs into windows 7 will be difficult and entailed to do. A very old machine may not be compatable, as it is slow.
Windows 7 is meant for brand new PC's, where you will see the most improvement.
BTW Trainz runs perfectly well on my Vista Desktop PC 3gb, 500mb HD, Nividia7800 Video Card
It also works fairly well on my GF 1gb Vista laptop, that has a factory video card.
You can try to polish a barney, but in the end you will only have a nice shiney barney !