You will not see much of an improvement using a HD6450 Crossfired with your APU. If you want to do Crossfire, save some more money and get a HD6670.
However, you would be even better off if you did not Crossfire at all and used a single, stand alone, video card like a HD7770. That would mean saving more money but performance would be better than a Crossfired APU+GPU solution.
If you insist on getting a HD6450, get the one with the faster DDR5 memory. Remember, the APU memory (which uses your motherboard system memory) and your GPU memory are completely independent from one another.