PCI is forward and backward compatible. Whatever you stick in will work at the highest speed both MB and Card support.
Look at cards other than a 6670. Getting the APU to help the graphics card is nice, but if you want good gaming graphics you'll want more than a 6670. (if your budget just covers the 6670 you'll find it's much better than the APU graphics).
You have a 300w power supply: Total wattage: 300W That will support about 100w of graphics. Something like an HD7770 will give you a lot of gaming w/o needing a new power supply. HD7770 uses 80w max. The hd6670 is about 63-66w.
Read this is you go with the HD6670 and dual graphics: http://www.eggxpert.com/forums/thread/753321.aspx