I would and am going to use two 4670 cards. Together they are cheaper and faster then one 4850. Plus they run much cooler. I have the same mobo and the x4 slot will not put any bottleneck that your notice from the 4670 but when moving to higher end card could be enough to think about.
That's quite not right. On Newegg, the cheapest 4670 cost $80 each, plus $9 shipping each. That's almost $180. The cheapest 4850 cost $170, or $140 if you count in mail in rebate, plus free shipping. Single cards do not bottleneck, so it's a non issue. A single 4850 outperforms 4670 cf. 4670 cf is a worse deal no matter how you look at it.