Hello,
from benchmarking on my r/o in memory database, i can tell that 9.1 on x5650 is faster than 9.2 on e2440.
I do not have x5690, but i have not so loaded e2660.
If you can give me a dump and some queries, i can bench them.
Nevertheless x5690 seems more efficient on single threaded workload than 2660, unless you have many clients.