Depending upon when you will be moving and your priorities for your kids, you may also want to consider Imagine International Academy of North Texas, a public charter school scheduled to open in McKinney fall of 2010. I'm already excited about sending my 2 yo DD there when she's old enough. Otherwise, she'll go to Caldwell, which I've generally heard good things about, although I have heard anecdotal comments about too much segregation between Spanish and English speaking children (there is a significant hispanic population on this--the East--side of town. I value the diversity, and want DD to learn Spanish, which is unfortunately not encouraged as much as it should be).
Also, as for the comment by the other mom about looking at middle and high schools, too, this is a good idea, but be aware that middle and high schools here are not zoned purely geographically--race and ethnicity are taken into account too. Your child IS supposed to go to the neighborhood elementary school, regardless. But by law all middle and high schools in town need to have the same racial/ethnic proportions--so, for instance, my (white) DD would almost certainly go to McKinney High, to which she can walk, while the (Latina) girls next door to us get bussed to Boyd.
And finally, depending upon what you're looking for and where you're coming from (I'm from New England), you may want to look at the historic district of McKinney--we LOVE LOVE LOVE it here. Cool old houses, very friendly and close-knit neighborhood, and you can walk downtown. GL with your move!