You can't crossfire the 7450 with the 6670, for a 300w psu either the 6670 or the 7750. In my opinion a decent upgrade will be for a 7750, which will run on a 300w psu, maximum tdp of the card is 55W, no auxiliary power needed.
avenseth12
You cannot xfire those cards. Here is a 7750 as an option:
I'd go with the 7750, so you don't have to replace your PSU. Otherwise you'll need to upgrade your PSU and then get a different GPU. Just make sure you get a DDR5 version 7750, if you go that way.