we keep them in stock... it depends on the bike but general rule is -1 on the front is the same as +2 on the rear

it really isn't enough to matter, but going smaller on the front will cause more stress on the chain like shane said, and more stress on the crank shaft and crank case, don't let that alarm you its so minut, but it is there... also going smaller on the front sprocket will make your wheel base longer, effectively making it harder to wheelie, but again this isn't enough to matter and adjusting your chain or taking a link out will more than off set this... a larger rear sprocket will shorten your wheel base making it easier to wheelie technically, if your going +2 it will be close, you might be able to get by all the way forward in chain adjustment but you may need a new chain... also remember a front sprocket is 15-20$, a rear sprocket is 40-60$

all those factors other than price really aren't enough to matter one way or another unless like shane said your going down more than -2 or adding like +4 to the rear... i've gone -1 on some bikes and +2 on other bikes and couldn't tell much of a difference... if you need a new chain i'd do the 520 conversion like Mister said, but if not i'd just throw a 19 dollar front -1 sprocket on it and see how you like that, like i said we have them if you want to give me a call tomorrow