Hi Playdo,
The following article tells you much about the RAM questions you are asking and the options and the consequences and the limitations and such. I would start here as a basic tutorial on the subject:
RAM - Random Access Memory - Types of RAM memory used in desktop and laptop PCs – Page 2 of 2.
See the following threads from other forums that discuss using RAM rated at higher speeds than supported by the motherboard. It seems that the RAM will work at the motherboard's maximum (even if it could be faster with a different motherboard) and BIOS plays a factor as well (if the BIOS supports that option):
RAM Max supported 333 mhz but you use 400 mhz Webmaster Hardware forum at WebmasterWorld. Here's another thread that essentially provides similar advice:
Can you use faster ram than your motherboard supports? For example if my mobo uses 240 pin 667 mhz ram can I use 240 pin 800 mhz ram? - Ask or Answer Computers & Accessories Questions - ibibo Sawaal Expert Answers.
That's why it's working - because it's running slower than the RAM could under diffferent conditions. If the motherboard maxes at 533, then even though the chips may be 667, they are almost certainly running at 533.
And you do need RAM that at least is the minimum supported by the motherboard - you should not use slower RAM (I'm not positive, but slower RAM may not even work properly - in effect, it may be "can't" as opposed to "shouldn't - but I'm not sure. To be safe, don't do it - you want the faster RAM anyway. You should use chips that are the fastest supported by the motherboard and otherwise compatible with the specifications of the MOBO, the system, the BiOS, the version of Vista, and the CPU and that do not exceed the maximum the system can handle in terms of amount.).
And when replacing or adding RAM, it is best to use the same RAM chips in the slots (though they can be different sizes as in one can be 512MB and another 256MB - as exists in an old XP computer I still use sometimes) - but the chips otherwise should be the same in terms of specifications. Don't mix DDR2 with DDR3 or anything like that. You should not mix 667Mhz with 800Mhz - but if you do, I believe the consequence will be that both will operate at the slower speed (however, I do not recommend doing this unless there's no other choice). And make sure you don't exceed the maximum RAM the system can handle (if it maxes at 2GB, don't install 4GB). If you add the same RAM chip you already have, you should be completely fine (as long as it doesn't cause you to exceed the max RAM for the system).
If you add something different (or change chips to something different), you may need to confirm that the system can handle that type of RAM and not just the motherboard itself but also the BIOS and your version of Vista and possibly the CPU - and of course you should comply with the motherboard specifications though, as noted before, you can usually install faster RAM but it really won't benefit you at all. Your motherboard isn't going to blow up or anything like that if you use faster RAM as long as everything else is within specifications (and if not otherwise within specs, then you may have problems or possibly damage something - but no explosions).
I hope this helps. I think the first article is best and will actually answer your questions, but the others will help in terms of confirming what you've read and seeing what others have done and recommended in similar situations. There are rules in terms of which slots to use if not using all of them for various types of RAM and those too are discussed. There are quite a few things to consider. In short, it's a great article (and I just read it and learned quite a bit myself).
Good luck!
