Mardi Gras is on Feb 28 in 2006, which is the week after President's Week. I think Disney usually considers that Peak Season, but they still usually offer some good AP and other discount rates during that time.
As for crowds, many schools around here (New Orleans area) are off all week for Mardi Gras, and it always seems like a lot of people down there then. It's become a joke that you may go years w/out seeing people you know who live in your town, but you always run into them at Disney during Mardi Gras! That said though, I've read websites that rank MG week a "4" on a 1-10 scale of how crowded the parks are. I'm sure the weather is nice, too, so I'd say go for it!