The Mac Pro CPU Compatibility thread front post will have any info on steppings etc. For the X5680/90 I don't see anything noted. The CPU model (X5680 etc) implies the "type" that you are concerned about, i.e. X5680 *is* a Dual Xeon.
I don't know about the RAM, but I will say that unless you are running CPU / memory bandwidth limited applications, which are pretty rare IMO, you'll never see the difference. I'd say don't worry about it, boot it up with pram/smc resets after you put the CPU's in, and see what speed the memory is running at. If it's still 1066 you can upgrade the RAM at your leisure but I wouldn't make it a priority by any means.
I don't know about the RAM, but I will say that unless you are running CPU / memory bandwidth limited applications, which are pretty rare IMO, you'll never see the difference. I'd say don't worry about it, boot it up with pram/smc resets after you put the CPU's in, and see what speed the memory is running at. If it's still 1066 you can upgrade the RAM at your leisure but I wouldn't make it a priority by any means.