Back surrounds can be a nice upgrade, but they are very dependent on room layout. If your seating is against the back wall, it may not be a space that is well-suited to upgrading to 7.1. If you have some space behind the seating position, you will be able to locate the existing surrounds to the sides and locate back surrounds on the back wall, an arrangement that will allow optimal results from a 7.1 system. You may also want to use direct-radiating speakers (like bookshelves) rather than dipole speakers for the back surrounds. I've done both, and have come to agree with direct-radiating rear surrounds (the recommended arrangement per both Dolby and THX).

The SR7002 will support seven speakers, and I think it has a decent amount of internal power. Adding two rear surrounds - unless you add something really exotic and use some all-channel stereo mode - will not create much additional load on the receiver's power supply. Adding the 7500 would be more effective an upgrade in improving what you have now (with or without rear surrounds) than anything else.