Though this question is not directly related to linux, still im asking this...
My motherboard supports DDR2 533MHz RAM. I have installed 1GB DDR2 667MHz RAM. It is working fine. I got a 256MB DDR2 667MHz RAM. I installed it in the second slot. But it is not getting detected...only the 1GB is getting detected.
If I install the 256MB RAM alone, then it gets detected...so no probs in the RAM or the slot...
What is the problem?
for DDR2 RAM is it mandatory to use two RAMs of same frequency?

You need to check your MoBo's manual for its RAM requirements. Search online if you have lost it.

Often RAM banks need to be same size. So if you have a 1GB bank, you can only add more 1GB banks.
If you have 4 slots you can get up to 4GB this way.
If you want 8GB you'll have to get 1x 8GB or 2x4GB or 4x2GB

RAM that is rated faster than your MoBo is OK.
RAM that is rated slower than your MoBo is NOT OK, unless you underclock your MoBo.

You need to check your MoBo's manual for its RAM requirements. Search online if you have lost it.

Often RAM banks need to be same size. So if you have a 1GB bank, you can only add more 1GB banks.
If you have 4 slots you can get up to 4GB this way.
If you want 8GB you'll have to get 1x 8GB or 2x4GB or 4x2GB

RAM that is rated faster than your MoBo is OK.
RAM that is rated slower than your MoBo is NOT OK, unless you underclock your MoBo.

My motherboard supports RAM from 256MB to 1GB DDR2 533MHz..

So ?

Please now read the bit in your MoBo's manual about what sizes of RAM can and cannot be mixed-and-matched in which sockets.

nothing of that sort is mentioned in the manual...
i think both the RAMs should be of same frequency and capacity,is it so??

In my experience, yes.

There should not be any problems using the 256 MB memory module. If there is any problems, it could be you have all slots filled up with double-sided memory. Not all motherboards can handle all slots with double-sided memory. Since you are using one 1 GB memory module and one 256 MB memory module, there could be a problem with the firmware or BIOS of your motherboard because it seems your motherboard requires two slots to be equal sizes to work.

Dual channel memory requires two modules be equal sizes and the quantity per upgrade should be in pairs. An odd number of memory modules penalizes dual channel performance. This is the same for triple memory, but each upgrade have to be three memory modules at a time of equal capacity.

I suggest return the 256 MB memory module with a 1 GB memory module. It should definitely work.

I have only two slots in my motherboard...and its specification says it has single-channel architecture...is that the reason for my problem?
can't i use two different types of RAM in single-channel?

Did you try the reverse positions? 256MB in the first slot and 1GB in the second slot?

When you have two different ram sizes, most motherboards are picky about the sequence and there is usually neither documentation nor logic to how they are picky. With just two slots, it doesn't take much experimentation to find out what is acceptable and what isn't.

I thought you said they were the same frequency, just not the same size.

I think the BIOS must select one frequency for all the RAM. RAM works fine at a lower frequency than it is rated. The BIOS should look at all the RAM and select the frequency of the slowest ram. Some BIOSs look at only some of the ram slots in selecting the slowest. So if the slowest is in a slot the BIOS doesn't check, the slowest ram will malfunction.

I assume you already understand that for dual channel, the ram must be in matched pairs. A dual channel capable system should be able to recognize that the ram is not right for dual channel and the BIOS will force single channel (which has slower performance). But I think you said your motherboard isn't even capable of dual channel, meaning it shouldn't matter whether the two pieces of ram are matched. But it still should matter that the larger one go in the position that the memory controller allows to be larger and it may matter that the slower one goes in the position the BIOS checks for speed and maybe those two requirements could conflict with each other.