MGS2 indded has 5.1, but just on the cutscenes, same goes for FFX (wich, oddly enough, DOESNT play in DD 5.1 while in game, but when you go to the place where you can "buy" the cinematics, those have 5.1!)
GT3 DOES have 16:9 support, but of course, it's low res as hell, same goes for GT4, widescreen support.. no progressive scan though (wich kinda sucks, cause my Projector is progresive scan 1280x720) it still looks "ok" but true HD PC gaming looks MUCH cleaner than that.
GT4 has been confirmed not to get progressive scan support, mainly because of video memory limitations, probably fillrate too (they'd have to get rid of lots of effects to make it work)
as for 5.1, this is as of yet unconfirmed, however, it *IS* doable, but it's very unlikley we'd see it because the CPU is allready being pushed to the limit, the are some games out there with 5.1 DTS out of the ps2 (DTS only, since it's less compressed than dolby digital, it's easier to encode on the fly for the PS2).
hopefuly PD found a way to get 5.1 with out taking a big performance hit, but don't hold your breath. BTW you haven't lived untill you played GT4 on a 16:9 72" screen ^_^