There's only handful of 3D games being emulated that I know of, and most of them are on the PSX anway, so is kind of a non-issue as far as this old gamer is concerned.
I have recently though, checked out some new arcade games... there's some 2D stuff that came out within the last year or so that I'd love to see (and I think the Xbox *MIGHT* be able to handle) but I know we won't see it.