Crossfire 4850@8x/8x PCIe or 4870@16x

I recently purchased a new monitor that runs at 1920x1080. I own a 4850 (512MB) running in a P45 chipset board. I like to run my games in as high of detail as possible (who doesn't).

Which situation would serve me better:

Purchase a second 4850 and run in crossfire at 8x/8x

or

Purchase a 4870 to run as a single card solution?

Thanks for any feedback.

^
+1

Get a second 4850 as two of them will generally preform better than a single 4870.