sparky,
Take her away somewhere *special* for her birthday - somewhere really special.. It doesn't have to be anywhere expensive - just somewhere that is special for the two of you.. Thats the 'present'. Then when you are ready you can propose to your GF in this special place. Take your time. Involve other people in the setup of your plan (of course swearing them to secrecy).
you only get to do this once. think about it and make it special for the both of you.
I hatched a plan and did the same.. it works..
good luck..
x