Cosmo, your spelling doesn't look so bad, but saying
P.S. I know my spelling is lackluster, I just don't really care.
is worse than not being able to spell. I have patience for people whose talent lies in things other than spelling, but saying you don't care -- and when asking for help or advice at that! -- is a sign of disrespect for your readers. Any time invested by you to clean your text of errors and typos is saved by those who read your text.
To reply to your questions, if you do get a 170, it wouldn't hurt to apply to higher-ranked schools. Assuming the rest of your application is OK, a 3.72/170 will give you a good chance at many schools in the top 10.
Your first question is unanswerable, I think. If you consistently scored in the high 160s, a 170 is not impossible, but I don't think we can say it is a "realistic goal" based on this information. If you feel that you are still improving from one test to the next, you might be able to win a few more points through additional studying, but if the high 160s is what you score all the time, you'd have to be lucky to score higher than you're used to precisely on test day. Of course it's not impossible, but don't count on it.