I just picked up a 2010 13" mbp and planned to do the same. Few things to note:
If you can get guaranteed matching ram from OWC. That's a guarantee it will work with your 2010. It needs to be able to run at the 1066mhz speed and to guarantee that it has to be mfg ram at that speed or programmed to be "1066 MHz" ram
I tried going my own way and ended up getting some ram that was speed locked so cannot do the downgrade. At this point I have a 4mb module + a 8gb one since the 4 is 1066 speed and the system will downgrade to the lowest common denominator speed.
there are some 1333 MHz modules that have worked for some people but it's hard to say. The process of reprogramming requires some software and a Windows partition or computer to be able to do it. You cannot use a VM within os X to do it
