The 4x slot won't help alot giving you a nice drop in performance. So the GTX 260 would be the way to go in this situation. If you had at least a 2 8x ports the 4850s might be more ideal.
I'd have to agree with rattle though, the 4850's are great in CF, when the drivers play nicely. Since I've been having more issues with it I went with the GTX285, yes its a hit in performance but its also faster in some cases and easier to handle. If I need more I'll just buy another for SLI.