650 wheels?

700 is a bigger number than 650....therefore, it must be better.

Ok, honestly, I have no idea, it's just the line of reasoning I hear all the time in my IT job. 

I did find a few articles with the Google though.

http://www.google.com/search?hl=en&q=650c+vs.+700c+wheels

Aerodynamically, the differences between the sizes are a wash. What you gain in a smaller wheel you generally make up for with a longer frame tube.

With the advent of integrated headsets, the need for the smaller wheel to get low enough in front is lessened, so now you only see frames in small sizes with the 650's.

My beef is that all this move to 700's is all about getting low in front, and ignores the overall weight balance on the bike and how it affects the bike's handling. A larger rear wheel pushes it farther back, which then requires a longer front end (usually longer than regulations allow) to balance it out. This is especially true if you ride anything north of, say, 78 degrees effective seat tube angle.

All my tri bikes (I have 3) are old frames that use 650's, and even of those, only 1 is really suited to an aggressive position, and is the one I race on.

As for tire selection, I've never seen a problem with the number of tires offered. But you'll find that there isn't anything wider than 23mm.