Re: PC 6400 vs PC2 6400
The "PC" naming convention doesn't seem to be particularly standard, as people are always putting typos, computer shops/online retailers are particularly bad at mixing this up.
If you have a DDR2 board, put DDR2 RAM in it regardless of the PC/PC2 question, as long as the motherboard says it supports the capacity and speed of the RAM then it's all good.
Yes you should be able to replace the sticks with varying capacity/manufacturer, the RAM will no longer work in dual channel mode (or quad depending on the motherboard), but practically you won't really feel the difference.
Athlon II x4 645 || 1TB 7200rpm HDD || EVGA GTX 650Ti OC || 8GB DDR3 RAM || Windows 7 Home x64
i5 4210M || 500GB Samsung EVO 850 SSD || GeForce 825M || 16GB DDR3 RAM || Windows 10 x64