If ME2 is the only game you feel obligated to play, I'm really not sure it's worth buying an Xbox considering there is a better PC version and you will eventually have to upgrade your computer at some point in time.
If you didn't play ME on the Xbox, then you lose the best reason to play ME2 on the Xbox; carry over game saves. Other than that, about the only reason for going with the console version, if you have a decent PC, is for the achievement points, which while valuable to many, is a pretty weak reason to buy the scaled down version for more $.
I wouldn't hold off specifically for a PS3 version since no mention has yet to have been made. On the wishful thinking side, for those who never played the original ME, odds would be with a late ME2 release, that you could reasonably expect to see it bundled with ME.
The game is story dense and drafted as a trilogy, so you would probably want to/need to play the original first anyway.
Either way, it's not like owning an Xbox wouldn't open up a bunch of other game options if you went that route; I just don't think it's worth buying for one specific game.