I would personally get the 4850 in your situation since it actually has slightly better performance over-all than the 9800GTX (and especially an 8800 GTS 512). Additionally, it supports DX10.1.
If they had used a 512bit bus it would have increased the production costs significantly and further raised the price of the graphics card. Additionally, due to the massive speed of the GDDR5 memory, it wouldnt have made much difference to its performance anyway.