m4a78-em will run am3 with bios update , but will NOT run ddr3 memory! As for m3a78-em, it runs ddr2 memory as well, but only runs in a green mode at 800mhz. it will run at 1066 , but only if you overclock it!!!!Which takes you out of green mode. Running 1066 Memory on either board will require am2+ cpu.
Also both boards are based on 780g which has onboard video output at 60hz. If you want more you may want to step up to the 790g series. Although it comes with a price jump it may be ddr3 friendly..
I suggest always checking the Qualified Vendors List or QVL , so you dont wast $$ on parts that wont work !!!!!! The m4a78-em , PowerDVD9 , XBMC ,a big screen and some good sound and you will love your HTPC
!!! I hope this helps ...