Well it'll show you in the mobo's manual and you could also look at asus.com or any other website that lists the specs on that board.
The furthest it'll go though is to a 3200+XP chip
Welcome to TF =)
*edit*
Considering that's a waste of money, you may want to look into saving your cash and upgrading to a 64b chip.
You could overclock your 2800 to speeds of a 3200 chip, there's really no point in wasting your money, for such a small increase in speed.