Can I install two slightly different CPUs in one system

I'd be inclined to choose the 2x nelaheim processors configuration. You've said you'll be running a lot of websites which can translate to a lot of web application threads, and 2 4 core processors translates to a lot of multi-tasking ability, and the throughput on the newest intel processors is much improved over their older processors. You should also see some very good memory performance from that architecture too, which should help with serving website content nice and quickly.

Intel's answer to stepping: http://www.intel.com/support/processors/sb/cs-001665.htm

In short, different steppings are from different manufacturing process and the higher the better.

update:

Having too much free time, I investigated a little deeper. Wikipedia lists the CPU families along with stepping numbers: http://en.wikipedia.org/wiki/Core_%28microarchitecture%29#Steppings_using_65nm_process

E5345 CPUs had two steppings (ever): B3 and G0. Looking at the Wikipedia site, the CPUID of those stepping are:

• B3 - 06F7
• G0 - 06FB

It looks to me that the stepping that is displayed in the CPU info table is actually the last hexadecimal digit (written as decimal) of the CPUID (which refers to the real stepping code).