Hi Yikes! Great initiative, wanting to get those letters out this early! But, I agree with the others that it is a little too early to make the connections. I'd suggest you wait until early August (before things get hectic with classes but late enough that profs will have an idea of whether they have an opening). I'd further suggest you spend the summer researching schools to apply to, if you haven't put a ton of work into that already, and find places with great fit with your interests.
For the letters themselves, read at least one study by every single prof you want to work with. Mention something (something *genuine*!) about your interest in the area, an idea you have for a spin-off study, or whatever. Profs get a million "Hi, taking anyone?" "Hi, taking anyone?" e-mails from student professing interest in them. Doing the reading will do more than profess interest, it'll demonstrate it! You're MUCH more likely to have a short back-and-forth exchange with the prof ("Great idea for a follow-up. But what about blah blah?") which will result in your being distinguished from the horde of prospective applicants. The really important thing to emphasize, I suggest, if that your interest in them stems out of a real, solid fascination with what they're doing in terms of their work and that it's something you want to pursue.
And, while you didn't ask about it, I'll provide the unsolicited advice to make sure to do this for **EVERY** prof you have on your potential list. Not finding out if the prof is taking a student can result in a waste of time making an application to a school. Profs take sabbaticals, they go on mat/pat leave, their grants don't go through and limit their # of students, they retire, and their interests shift. You MUST contact every prof by e-mail before you apply!!
