I'm guessing that MS Train Simulator isn't the most demanding of games graphically so I've just gone with a 7600GT but you could fit an X1950pro into the budget. There is still scope to enhance a few parts within the budget but it is more than sufficient currently for the tasks you describe.
Logitech Cordless Desktop EX-110 - Retail £17.99
(£21.14) £17.99
(£21.14)
GeIL 2GB (2x1GB) PC6400C4 800MHz Ultra Low Latency DDR2 Dual Channel Kit (GX22GB6400UDC) £94.99
(£111.61) £94.99
(£111.61)
OcUK Value Hanns-G HW191D 19" Widescreen LCD Monitor - Silver £94.99
(£111.61) £94.99
(£111.61)
Antec NSK6500 Super Midi Tower Case - 430W PSU £58.99
(£69.31) £58.99
(£69.31)
Intel Core 2 DUO E6300 "LGA775 Allendale" 1.86GHz (1066FSB) - Retail £99.99
(£117.49) £99.99
(£117.49)
Gigabyte GA_965P_S3 (Socket 775) PCI-Express DDR2 Motherboard £59.99
(£70.49) £59.99
(£70.49)
OcUK GeForce 7600 GT 256MB GDDR3 TV-Out/DVI (PCI-Express) - Retail £56.99
(£66.96) £56.99
(£66.96)
Seagate Barracuda 7200.10 320GB ST3320620AS SATA-II 16MB Cache - OEM £51.99
(£61.09) £51.99
(£61.09)
Samsung SH-W183 18x18 DVD±RW Serial ATA Dual Layer ReWriter (Black) - OEM £17.99
(£21.14) £17.99
(£21.14)
Creative SBS 350 2.1 Speakers - Retail £24.95
(£29.32) £24.95
(£29.32)
Sub Total : £578.86
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping : £14.95
Vat : £103.92
Total : £697.73