First of all, Sapphire. Not Saphyre.
Second of all, the memory bus is measured in bits, not megabytes. (the bus measures how fast the card can use the memory)
Third of all, just because it has more memory does NOT mean it's a better card.
As a matter of fact, the 4670 has a narrower bus than the 3850. The memory is clocked at a bit faster speed, though.
The 4670 may handle Anti Aliasing a little better, but for the most part the cards will perform the same.
The 4670 is not a powerful enough card to handle that much memory anyway. It would have made hardly any difference to get a 512mb version. It's all about the stream processor count, and those two cards have the same number of stream processors.