I actually went with Lexar RAM (just upgraded last week actually) and on the box it was listed as BOTH 1066 MHz and 1333 MHz. My system (mid 2009 2.53 GHz C2D) is listed as using 1066 MHz. So I knew the RAM would work either way. The RAM should have just defaulted to the 1066 MHz.
What I find interesting, is that in System Information, it says it's running at the 1333 MHz! If my system can only utilize 1066 MHz tops, then why is the RAM actually running at 1333 MHz? Shouldn't it say 1066 MHz under System Information?
Anyway, you CAN use higher speed memory without any issues (of course its possible you could also have issues too) I am using higher speed RAM without issues... Although I don't know if it's actually using the extra speed or if it's just listing it in System Information. I have not had any issues at all though, aside from needing to reset the SMC once I installed it. (I started having some sleep issues which started after installing the new RAM, and resolved as soon I reset the SMC)
If the high speed RAM costs more though, I wouldn't bother paying the extra. The Lexar RAM I bought was both speeds... and there was no cheaper alternative locally in stock (not that it was expensive either - it was about 100.00 CDN for 2 x 4 GB sticks....comparable with online prices LOL)