Radeon 256bit bus width vs 128 bit question

Hi, I am hoping someone can help me with some information. I have Biostar mobo TA790GXB A2+. I have at the moment installed an AMD Athlon™ 64 X2 Dual-Core 5200+ which I plan on upgrading to a phenomII x4 in a few months. Here is my question, I would really like to purchase a new video card (its my birthday:D). The 2 I am deciding between is

SAPPHIRE Radeon HD 6850 1GB 256-bit GDDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card with Eyefinity (100315L )

or a

SAPPHIRE Vapor-X Radeon HD 6770 1GB 128-bit GDDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card (100328VXL)

My question is this: The processor speed and the memory speed on the 6770 are certainly better but as the Radeon 6850 has a 256 bit bus width it is a faster card however, would my mother board be able to actually utilize all of the 256 bit bus width? I was told by Best Buy that with an A2+ motherboard I would be better off going with the Radeon HD 6770 with the 128 bit bus width since I would not be able to use all the 256 bit width with my mobo.. I have tried checking with Biostar but they dont seem to know if this is true. Can anyone tell me how I can verify this as fact? Or is this just an opinion? Theres no point in buying the more awesome expensive card if I will not be able to make use of all its awesomeness. Thanks so much for any consideration you give my question.
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  1. The HD6850 outperforms the HD6770 in your board as well since the memory bus is internal on the card between GPU and Vram and has nothing to do with your motherboard!
    The correct way to compare GPU's is benchmarks HD6770 performs on level with HD5770 http://www.techpowerup.com/reviews/Powercolor/HD_6850_SCS3_Passive/27.html
  2. Thank you so much for the information as well as the link. You helped make my decision a snap. I appreciate your time very much.
    Thanks again
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