With your current budget I think keeping your receiver would be the way to go. Unless you really want a 6.1 or 7.1 I feel it wouldnt be a good idea to get a new receiver in the $500-700 range.
Spending $700 at most you are only going to get a reciever with around 100x7 watts. 100 watts in a reciever is not much more than your 75.
You said that your preferences are 80% movies. Maybe you should invest in some surrounds. That would probably improve your home theater experiance a lot more than getting a new receiever. Used 602's or 601's go for cheap on e-bay and audiogon.
Even a more powerful sub would probably be a better upgrade for you than a new reciever. With $700 you could probably get a really impressive sub.
I feel that in you situation the most logical upgrade would be to add some surrrounds, or purchase some speakers to replace your 602's and make them perform surround duties. The few extra watts that would get from upgrading your reciever would probably be barely noticable, if at all. The only reason I would get the reciever is if you want 7.1 or 6.1, but you don't even have a 5.1 right now. You might find out that you are happy with only a 5.1 and don't need more.