The cores refer to differences mainly in the L2 cache.
The wider question is much more difficult to respond to. First, in order to get the best out of a 64 bit processor, you need a 64 bit operating system. XP (64) has only just been released and is only available as an upgrade or to OEM manufacturers. You may find some sites able to offer it with hardware. Linux has had a 64 bit capability for sometime, but cannot be considered a serious gaming platform. There is widespread concern over the lack of drivers currently available, especially for older equipment. So printers, scanners and the like, may have problems communicating with the system. There are few programs written (at the moment) to take advantage of the 64 bit capability, although over time, that will improve.
The main reason for 64 bit, is to improve the amount of memory the system can support (up to 4GBts I think), which is important in a server situation, but will make little difference to the current crop of games.
In conclusion, I would doubt at this moment in time you would see any significant difference in your games. Until this process has had time to develop, and more software is written specifically for its use, I would be on the side of saving my money.