Sure, the Xbox is able to emulate any of the NeoGeo ROMs, but there's a little problem, the RAM.... If you ever played a NeoGeo AES you should already know that the NeoGeo carts are ROM carts with huge amounts of data, and, like any cart-based system, the connection between the system and the cart is extremely fast, no loading times, etc... in other words, the ROM cart becomes the system's RAM (in a way)... This means that the NeoGeo ROMs that you load into the Xbox's memory have to fit into the Xbox's RAM, fully uncompressed... so that the system has full access to the games' data.... the only way to accomplish this would be:
1. Modding your Xbox for it to have 128Mb RAM.
2. Porting a NeoGeo CD emulator.
If you mod your Xbox and turn it 128Mb you would prolly be able to play ALL the NeoGeo releases, why? Because the NeoGeo carts cannot hold more than 1Gbit of data, which translates to... 128Mb... Anyway, the biggest NeoGeo ROM available is King of Fighters 2001, which is 892Mbit, that means, 111.5Mb.
Porting the NeoGeo CD emulator would be useful, but has a drawback.
Each NeoGeo CD load wouldn't use more than 7Mb of the Xbox RAM, this means you would be able to play any of the NeoGeo CD games with ease, the problem is that these games have CDA format audio... so, you would only be able to fit one game per disc... and, the other problem is that there have been no NeoGeo CD releases since 1999... that means: forget about new releases.
So, here's the thing, you have to choose: getting a emu with the latest releases, but with some limitations (your own Xbox's RAM), or getting an emu that lets you run all games smoothly, with CDA quality audio, but using one disc per game and not getting the latest games.