05-31-2012 05:57 AM - edited 05-31-2012 05:58 AM
this KB article should help: /t5/ThinkStation-Knowledge-Base/Supported-processo
any processor listed that doesn't have a red asterisk next to it is supported in the C20. you would need a C20x to use CPUs with a 130W TDP.
the heatsinks used on your E5620s will work with everything up to an X5675.
06-12-2012 10:18 AM
unfortunately mismatched CPUs won't work together. there wouldn't be anything to gain even if it were designed to work since the lowest-common-denominator rule would kick in, allowing the slowest device in the chain to set the pace for everything else.
on the CPU itself, another option would be the X5650 or X5660 if budget doesn't allow for an X5675. i've had all three in my C20 and couldn't tell much of a difference between them. the X5650 is arguably the best value in a high-end 2QPI westmere CPU and would provide an affordable upgrade path if you decide to expand to a second X5650. the E5645 would be top choice in the mid-range.