The speed is how fast any operation on the RAM is clocked - not how much bandwidth is available to be shared amongst each bit of the module - so a 2GB module clocked at 667MHz should (all other things being equal) run at the same speed as a 1GB module clocked at 667.
If your machine's chipset supports dual-channel memory access, and the two memory modules are properly matched and recognised as such by the machine, then 2x1Gb may be marginally faster. While not an entirely accurate analogy, you could think of dual-channel RAM as RAID0 for memory. Most people don't really notice the difference though - the most memory bandwidth hungry things you are likely to ask the machine to do will have significant bottlenecks elsewhere, such as graphics performance or the speed or your CPU (and the CPU's cache).
Check to make sure you know what your machine can take before buying: look in the machine's manual or the databases maintained by memory sellers like Kingston. You might find it can only accept up to 1Gb per slot in which case the decision is made for you. If your machine's official spec is "max 2Gb" and it has two slots then you can pretty much guarantee it won't accept modules larger than 1Gb.
Also, be aware that you have a limited number of slots. If you plan to upgrade further later then getting the larger module now and leaving slots free might be more convenient.
