Umm... that makes little sense though... emulate PC and then emulate SNES/MAME? The PS2 has a 333mhz processor, 32mb of ram. That's really what you need to emulate on a PC, but it's in my experience, the bare minimum. If you're emulating the PC too... urg... that'd be so slow.
And the XBox will probably always have better emulation because it has better capabilities (hardware). The processor and RAM are what? Twice that of a PS2. And the XBox's ram is DDR. So yeah...
ALLERGY ALERT: The above post may be nuts.
N64, GBA, both un-modded.
NTSC U/C PS2 v4 w/No Solder v4 (SOLD, DRE... Laser Died
NTSC U/C PS with Plugmod (SOLD to pay for above