first up a comparison of bulldozer 8 core with a Vishera 6 core looks like this: http://www.anandtech.com/bench/Product/434?vs=699
if you focus on games, you would see the 6300 trumping the 8150 simply because the vishera is more powerful and better tweaked core than bulldozer. the only reason the 6300 gets trumped at times is a comparatively lower clock and the obvious lack of a compute module (which AMD call "two cores").
Now it is safe to run AM3+ cpu's on AM3 boards but you must remember that AM3+ cpu's and sockets are designed for higher peak currents @ 145 amps compared to 110 amps that AM3 socket can supply. This will all mostly translate to the limit of OC'ing your board may permit.
to use a 6300, you must update to BIOS version P 2.10 as suggested here: http://www.asrock.com/mb/overview.asp?cat=CPU&Model=880G
use your older phenom and update the BIOS from here: http://www.asrock.com/mb/overview.asp?cat=Download&os=BIOS&Model=880G
Go ahead and get that 6300 and enjoy a great gaming build.