I've answered my own question, which turns out to have been embarassing.
I got frustrated because I couldn't find the answer. The answer is so well known that few people are talking about it much on the net. As a result unless you already know what to look for the answer is harder to find.
I had become so used to saying D-D-R for DDR ram that I had completely forgotten (and never really thought about) what it stood for.
DDR stands for 'double data rate'. This means that while the RAM works at (in this case) 800Mhz, it effectively delivers performance equivalent to 1600Mhz.
If anyone feels the need to add anything then feel free. Otherwise this thread can be shut down.
Well thank you for posting anyway. Good to know I'd have been told the answer even if I hadn't found it out.
I can't give myself 'best answer' so you'll get it. Yours was more concise anway!
Edit: hang on, before I give you best answer: is your use of the term 'base clock' really correct? Base clock should surely be somthing like 133Mhz (on a Nehalem chip for example). Is it really correct to refer to the 800Mhz aspect of the memory as the 'base clock'?