Both have a maximum, though with modern GPU's being as awesome as they are, it's usually the monitor that is the limiting factor.
There is room for an upgrade if you don't count shipping. I think you get charged tax up there as well. I wanted to leave room for that. If you want to upgrade something, there's nothing stopping you.
I'd suggest adding in an aftermarket cooler to make overclocking easier. I haven't really read up on which ones are good, though.
edit: There's a $290 dollar difference between the i7 920 and the i7 940. $290 for an extra .27GHz. It's not worth it. The 965 isn't sold in Canada apparently, and the 975 is $1140. If you were to upgrade, the OEM 960 would be your best bet, but you'd have to get an aftermarket cooler...OEM cpu's don't come with stock ones. That's still about a $370 difference though, including the cooler.
5850's start at $299, and 5870's at $412. That 5770 is overclocked, and is $196.