The 1.6 Ghz is the speed that each of the cores is capable of running at.
It isn't easy to directly compare a dual core to a single core even if the single core chip has double (and a bit) the speed it may have much older architecture a smaller level 2 cache, etc. So even though it has a faster speed it may not be as good as the dual core chip.
The dual core is also better at the multi tasking as one core can be doing one thing whilst the other does another. Whilst this is also dependant on the software having been optimised for dual/multiple cores.
When more software is designed to take advantage of this then the quad cores will rule the roost. Until then the dual core is fine and the motherboard may also be able to support the quad core, you may however, have to upgrade the BIOS.
So the choice is simple, dual core over a dated single core, as they would probably offer a more cost effective upgrade path also.