Xeon LGA771 vs LGA1366, Are 2 Quad Core of say 2.6 Ghz = 1 Quad Core of 3 Ghz

Hi,

I am building a server and need advice on which processor to purchase. I have a few questions:

1. Are there any performance and speed issues between the Xeon LGA771 and LGA1366 Processors?

2. Is 1 x LGA771 Quad Core 3 Ghz Processor faster than 2 x LGA1366 Quad Core 2.2 Ghz Processor?
(1 Nos. x Intel E5450 3.0 Ghz > 2 Nos. x E5520 2.26 Ghz) Please Note: The above config costs the same.

I will install Windows Server 2003, Exchange 2007,Other Small Applications like I.e. Accounting Software, Scanner Routing, Fax Routing. This Server will also be the domain controller.

I Intend to Install Windows Server 2008 in the future and would like the system to be future ready atleast for say 3 years.
---- I Expect the system to give fast responses when opening applications and excute the tasks rapidly-----

Please advise.