How fast would Dual 2.66 GHz E5430 QC Xeon processors?

The rated speed is 2.66 ghz and that's pretty much an average speed for a cpu. The added cpu then makes the whole process keep up to that rated speed since your adding more cores and the Pc will now not have to slow down to do all the processing through one cpu.
If your now talking upgrading to a higher cpu then the same will hold true your adding cores to do the work and keeping up the speed. The issue with a single cpu is that you have just the one and while it may have multiple cores and threads your adding the same amount to help withthe work to keep the cpu from bogging down.
Upgrading is a personal preference and it will depend on what your work load is and how fast you need it done so if you need more speed you can either overclock or get a higher stock speed.

It sounds like you are looking at possibly buying a machine but have not done so yet. I would personally not buy this machine unless you can get it for nearly free. The E5430s were decent processors in their day but they are five years old. Two E5430s will at best perform like a decent single modern quad-core Core i5 or a six-core AMD FX and at worst slower than any of them since the E5430 does not have any form of Turbo Boost and those other processors do. One E5430 would perform like a Core i3 at best. There is some limited upgrade ability for that motherboard. The fastest CPU that will definitely work is the 3.00 GHz quad-core E5472, which will be only about 10% faster than the E5430. I would skip the upgrade. The biggest issue with this machine is the chipset that it uses. The i5400 chipset this machine has uses FB-DIMM memory which runs very hot and is extremely expensive to buy, particularly the DDR2-800 FB-DIMMs the T7400 uses. This memory also has very high latency and kills the performance of the CPUs. The shared frontside bus architecture of the i5400 adversely affects application scaling when adding more CPU cores.

So in short, look at a modern machine and leave this one for the history books unless it's nearly free. If it is nearly free, use it as is and don't upgrade anything because it will be very expensive and not give you much of an increase in performance.