Hmm - if you move off GM6 to GM7, you lose Win98, but you gain Vista. The cross-compatibility tool does indeed make the entire range available, I think, but as you said it's not an option due to the Instant Play joy. So it's a choice between those two OSs, as far as I understand it. Whichever one you think is likely to generate more plays.

Still playing it every couple of days, by the way. :P

I think there are more people using Vista than Win98, at least in the developed world, so if I had to choose I'd go with Vista.