PC's RAM memory explanation

[Antonio Fernandez-Vicenti <afvaiv@xxxxxxxxxxxxxx>]

I've been googling to clarify some doubts, but the only thing I find are 
lots of commercial stores that sell all types of memories.

I understand that if I'm talking about KVR333 or KVR400, it has to do 
with internal speed. Hope I'm right at this.

Next, labels show X64. Does that mean it's only for PC's running 64 bits 
apps? Else, what does it mean?

Next, labels show C2 (e.g. an old one KVR133X64C2/256... ... old one and 
X64?), or C25, or C3A
Seems C3A are 1GB, whereas C25 are 512MB, but have some C25/256...

1- Can anybody give me some link to where all these codes are clearly 
explained?
2- Can I mix KVR400 and KVR333 on same PC? If yes, am I wasting 400's 
speed in the whole lot (would all run at 333?), or is the difference 
negligible or even better since I'm adding additional (333) memory to 
the 400 dimm?

Thanks,