Yes, due to the similarities in both hardware and APIs, moving pc emus to xbox should be pretty easy.
I've never had very good luck with the software emus, but got the best results from Proj64.
I'd venture that if you had the top 3 emus (Proj64, UltraHLE, Nemu) all tweaked (custom settings for various games)... you'll get like 30%+/- of all released titles(roms) to work.
The low success rate is what prompted me to just buy the Z64 (add-on for N64) which gives basically 100% compatibility ( exclusion of about 2-3 titles which are of the 512mb variety ).
Z64s can be had for about $150-175 now used, and allow you to play 200+ N64 titles ... pretty good investment vs. time/hassle/incompatibility of the pc emus.
IMHO. ;-)
Bleem! however, as playstation emu... seems fully compatible and truly gives great graphics/speed ( graphics are truly rendered/mapped and make psx games look closer to N64 quality ...on the pc).
Cheers,
-dp
P.S. re: pc-emu speed(vs xbox) 2 yrs ago when I ran the emus on a 550mhz K6-II, the frame rates were a little slow... however now with 1g+ Athlon it's great... and I image the speed of the xbox + nvidia would give great emu frame rates.