Couple of questions in here - without going to specific brands of servers any modern server is in most cases going to be able to easily cope with 5 VM's of the spec you describe. It does really depend though on the workload that the current servers are under. Do you have any data on this, cpu usage, disk i/o and network are the first parameters to look at. Basic system monitoring tools should give you some ideas in this area. To give you some idea we normally see between 4 and 6 VM's per core. So on a modern server you may easily see 30-40 VM's of typical workload profile.
Your cores question is more straightforward: In the old days CPU == cores, lets say each CPU is one engine and can do one thing at once ignoring things like pipelining etc. In this older model every chip essentially has one engine.
More recently the processor vendors have not been pushing clock speed up but putting more 'cores' on the same peice of silicon. So now each chip really has multiple CPU's or engines, this is confusing as some people refer to the CPU as the physical device that you buy from AMD or Intel. To get around this terminology problem we tend to refer to the physical device as either a 'package' or 'socket' to empasise we are talking about the phsyical device you plug in to the motherboard. So your old two CPU server has two physical locations where you can install 'processors'. These days each processor will have multiple cores/engines 2,4,6,8 etc. So your old 2 CPU box could have 16 cores or engines at its desposal.
Now couple of thinks about your question:
Lets say you have a modern 2.8 GHz (multi-core) processor and your comparing against an older 3.2 GHz proc (single core)
Firstly one of the 2.8GHz 'cores' is the doing the same function as your entire 3.2 GHz processor, you might have say four cores in the newer device.
Secondly you cannot make a a direct comparasion between a modern 2.8GHz processor and a 3.2 GHz processor from 3 years ago: they are worlds apart in their performance, even if you say you are only using 1/4 of the modern part.
Lastly the performance of multi-core cpu's cannot be aggregated in the way you say - 2 cores x 3GHz may not be equivalent to 6GHz. Depending on your application it may only give you 3GHz or it may approach 6GHz if your application is very scalable, in most cases it wont be.
