The guys were right for going with a dual core CPU.
Cheaper, and can reach high speeds on air for more performance per core.
This is most important in games, as even newer games like Crysis see negligible performance gains with similarly clocked quads vs dual cores.
As for the topic of the operating system, I don't believe it should be included in the price, however there should be some mention of the fact that if you do not have one handy to put on this computer (Stating that it would have to have its own license, etc) that you would also need to get an OS for the rig as well.
While I don't condone piracy, it is a big part of the industry, whether you like it or not, so it should be considered (and in some cases people may just not need to go out and buy a new OS).