First thing is to make sure you have the correct form Factor.
DIMM = Normal Large DDR2 Sticks used in PCs and Mac Pro
SODIMM = Laptop size RAM used in MB, MBP, iMac and mini
The difference lies in it's CL rating (Cas Latency) and Bus Speed. The lower the CL Rating, the faster the memory, the higher the cost. CL2 is high performance and i've seen up to CL5 for standard RAM.
Then there is the speed rating in MHz for DDR2. You can always buy faster RAM then your system will support but it will not perform at that speed, it will downclock it to what your system runs at. Your system memory runs at 667MHz. The expensive stuff sometimes is rated at 800MHz and would downclock to 667MHz making the extra money spent worthless.
Ultimatly, what you are looking for is SODIMM DDR2 667MHz (PC2-5300). Compare prices on different sticks and get the ones that are the lowest CL rating for the ammount of money you are willing to part with.
-MikeM