A is the fastest, running at PC2 6400 - 800mhz
B and C are slower, running at PC2 5400 - 667mhz.
The PC2 5300 vs PC2 5400 is just a naming convention discrepancy. Some vendors call it 5300, some call it 5400, but both sticks run at the same speed of 667mhz.
You are limited in the following ways:
1) Your describing only a single stick of ram. This means it is not paired, and will not run in dual channel mode. This will cut your speed in half. You should always run DDR2 ram in pairs for best performance.
2) You will be limited by your motherboard. If your motherboard can't handle PC26400, you can't put PC26400 sticks in your system. Make sure your motherboard supports the ram you are going to put in it.
