Sorry where did I say XP required 256 mb to run? Most operating systems use virtual memory or a swap file so physical memory and what is required doesn't always match but if you can keep out of the swap file then you don't get hit by the disk accesses. Memory is normally measured nano-seconds, hard drive in milli-seconds. There are 10,000 nano-seconds in a millisecond. There is a stripped down version of XP used in embedded system that uses something remarkably small for memory that's kind of fun.
Editing the .exe basically does the same as setting the compiler options however I would prefer to do it in the source code and compiler settings rather than core alter the executable. Probably after spending six months of my life reconstructing an assembly language sales ledge program that had been core altered a few times to save the time it took to compile. That was fine but then the programmer left and there was no source or documentation available. This is an option for Trainz by the way but whether Trainz could take advantage of it would have to be a subject of experimentation. I think either anandtech or tomshardware had a write up on this a year or two ago which has been referenced in the forum.
On the Vista comment well yes that is what I said in paragraph three a 32 bit operating system can run programs of more than 2 gigs under some specific circumstances.
Hardware and operating systems were one of my specialist subjects in the years before Windows, I think we had the dubious reputation of having the most heavily modified Geogre III operating system in the country, but we used to get twice the performance of other sites.
Cheerio John
