The last Best Buy console launch I did was for the 360. Went to the midnight launch and they allocated 30 for the evening and 30 for the morning.... I got there at 8pm. Didn't get one at midnight and a TON of people left. I stayed, along with a few others and there were 7 of us until 3am when more people started showing up.Try and ask your store about how many units they'll have extra. You might have some luck and get info, but maybe not. Check if they're doing a midnight launch, get there early and ask employees how the line system works for the extra consoles and just hang out.
For morning launch, as early as you're willing to wake up.
Tonyxvx has a great idea.... if you can get a # of consoles, you could possibly figure out if you will get a midnight system or not.