If I remember correctly even the AMD Sempron provides smooth blue ray playback with a good video card. (Single core in case you didn't know)
Dedicated Video Memory isn't important comparing 512mb vs 1024mb, which video chipsets are these? It sounds like the duel core is probably the better choice though for what you do, you'll see more overall performance with a higher clock speed in your computer.
Blu-ray playback is easy on a system. If you want FLAWLESSLY then get a PS3 or dedicated player. As long as PowerDVD is the only way to watch Blu-rays on a PC then your going to have to live with a few difficulties.