I think 2GB is the largest card capacity you'll ever need, and it might be the largest that the 2610 will accept. A lot of early CF products like cameras had a 2GB limit. The entire CN2009 product fits on my 2GB CF card with plenty of room to spare.
A very bottom of the heap-spec'ed SanDisk card and the Extreme-III version are equally fast in my 2610 as far as I can tell. Differences I was looking for were in the initialization at power-up, searching for points of interest, and staying current on the display as I travel.
Garmin has told me that there will never ever ever ever ever ever, ever
be any more up to date mapping product beyond CN2009, in the pre-NT format that the 2610 needs. Amazingly, when you go to the Shopping page on Garmin's site, they don't state what version they are selling, at least not that I can find. Even the picture of the product doesn't show a version number. CN2008 was stillborn, it was shut down as a product within less than a day of being released. But that didn't stop them from shipping me one when they were supposed to send 2009. I have two of them now so I'm all set for when a restaurant table leg is a little short. They say that they won't unlock 2008. "Ever".
Mexico isn't part of North America according to Garmin: