i'd imagine the xbox wouldnt be able to emulate it very fast. the ps2 is very different in tech specs to the xbox. dunno if you guys are tech savvy or not. the ps2's system bus (read. thing that all information is passed through) is alot wider than the xbox's..
I own a modded PS2 and before buying that I looked into this myth. A PSone emu. is already available for the Xbox, along with N64, SNES, NES, Genesis (mega drive), MAME, and a few others. However, at the moment PS2 emulating would be possible IF you want to play the whole game like its in super slo mo.
The Xbox has a faster processor than the PS2, but the PS2 has more RAM. Thus, only the teeny weeny PS2 games could be possible to emulate on the Xbox... but I still doubt it. I have contacted numerous mod guys and all of them so far have said that it would be near impossible to get it working smoothly if at all.
Some people even think that the N64 emulator runs far from perfect and that the only games that do work fine is Mario 64, and Mario Kart.
Unlike computer games, Console games are written around the console itself, with its limitations in mind. So something that runs well on a PS2, might run like a dog on Xbox, and vice versa (of course if it was at all possible).
The people who write the games obviously utilise the particular consoles equipment (ie faster processor for the Xbox, or bigger RAM for the PS2) to make the games run smoothly.