Have a look (search) on Intel Xeon E5-2670 thread, many people were buying
P4216XXMHGR for $100-$115 and pairing it with the Intel Server Board S2600CP motherboard/CPU deal on Natex. This would require from my understanding an Intel FCPUPMAD Air Duct Required for Intel Server Board S2600CP In P4000M. I don't know if you still need the air duct with another motherboard?
If using another motherboard, check the position of socket of power lead and see if PSU cabling will reach. There is few good documents on intel website regarding various intel chassis and accessories. e.g.
Intel FUP8X25HSDKS Spare 8x2.5 Hot-Swap Drive Cage Kit which they accepted $40.
Intel FUP8X25HSDKS Spare 8x2.5 Hot-Swap Drive Cage Kit New | eBay
Please note that Intel P4308XXMFDN Server Chassis 4U, 460W has 460watt PSU and could be limiting if you are running dual CPUs and lots of drives.
NB. There is a range of chassis available, with 2.5" bays or 5.5" bays, some with two mid-chassis fans and others with 5 mid-chassis fans, some models with 750watt PSU and others with 1200watt PSUs.
I hope this helps