You didn't say wich version of Trainz you are running.
But from what you have posted the specs will easily run any version Trainz. Don't worry about only going for a 9800GTX+ graphics card, the latest GeForce 250's are only re-badged 9800's.
I have a similar setup now, specs as follows:
Intel Core2Dua E6300 (2x1.86ghz)
Asus P5QL PRO Mobo
4 Gig OCZ DDR2 Dual Mode Ram
WD 250GB Sata II HD
Samsung F1 640GB Sata II HD
768MB GeForce 9600GSO
S/Blaster XFI Extreme
Windows Vista 32Bit Premium
Napalm Industries Rail Division