If your extension expires before your PR application is finished processing, you can apply for another, or return to the US for a vacation (no specific length required) and cross the border again. However, should you choose to cross again once your application has been submitted, it's highly recommended that you do so WITH your sponsor, and bring proof of your application (e.g. receipts for the processing fees and shipping, some even recommend keeping a copy of your application with you in case you get a border officer who wants to see proof that you're in a qualifying relationship). That said, it's generally easier and less stressful to apply for an extension from within Canada, as there's less risk of being refused by an employee who's having a bad day.
You probably already know this, but I just want to make sure you're clear about your initial visit: It's only a visit, even if you plan to stay. Whatever you do, do not tell the border officer that you're moving in with your boyfriend in Canada. This makes you a serious overstay risk, and you'll most likely be refused entry, because you can't move to another country without that government's permission. Until then, you're a visitor, and you'll need to be able to show significant ties to the US in order to prove that you can and will go back after your visit is over. Of course, I understand that this is difficult if you've in fact severed ties in the US in order to be with your boyfriend, but be prepared with as much as you can, including proof of your ability to support yourself during your stay without working illegally. If you're traveling by plane or bus, buy a return ticket for a reasonable date (i.e. within a few weeks of your arrival). There's no law against changing your mind later and cancelling that ticket in order to stay longer, as long as you don't stay beyond the date stamped in your passport. Showing up and saying that you want to stay for 6 months will make them very suspicious of your intentions. Most people also recommend avoiding mention of a romantic relationship, instead referring to your boyfriend as a "friend," though others have had little trouble with this.
Hope this helps!