Yes,
swap is utilized when the server ruins out of ram. I'd advise going dual CPU, if you're buying the hardware it'd be a total waste to find out that you need more later.
Going from two quad cores to one quad core might cause issues down the road (you basically have eight cores clocked at 2.4 and you're looking to move to 4 cores in the 3.5 range, if you go dual you will definitely be future proofing.
Have you logged in to the server and looked at resource usage? If this is a server that your host is assembling and no cash is coming out of your pocket then it's easier to try a configuration.
