Till now I too was under an impression that I can place single module and leave free slots on the computer for future expansion but recently I then came across an article that two RAMs are better than one.

The article said that two smaller RAM sticks will perform better than one larger stick. - Because the PC can access both sticks in parallel, so your computer can (theoretically) have access to twice the RAM as it could if you had only the single RAM stick.

No wonder in earlier days vendors use every possible RAM slot in order to take advantage of this effect. It wasn't uncommon to see a server with eight RAM sockets, each featuring a 128MB RAM chip, for a total of 1GB of RAM.

Personally, I tend to buy the largest RAM sticks I can afford when I'm upgrading a machine. Since most PCs have two slots, a typical scenario is finding a PC with one 256MB stick already in place, and I'll just add a 1GB stick to that to upgrade the RAM to 1.25GB, rather than remove the 256 MB stick and replace it with two 512MB sticks. If I'm configuring a new PC, I usually just go with what's cheapest, and a single 1GB stick will be about 10 to 15 percent cheaper than two 512MB sticks.

I think there is something called dual memory access, something like that. My motherboard, an Intel D915GAV have that. I'm pretty sure other recent motherboards would have them as well but since I'm not a hardware retailer, there's no way to be perfectly certain. You should check your motherboard. What this "dual" thing does is that if you put 2 "similar" RAM modules on the specified slots, then it would work better than just 2X the RAM modules on one slot. Some even say that this "dual" thing works twice as fast. You can go check it out on the internet for more info. Just pointing it out here for you.

This is true only for motherboards that support dual channel memory. Also, the two memory DIMMs need to be placed in appropriate slots for the full effect to be obtained. Although, nowhere near double as fast, dual channel RAM controllers will easily result in 20-30% increase in efficiency.

For dual-channel memory to work, both DIMMS must be of identical capacity and operating frequency - you can't expect a 400 MHz/256 MB DIMM to work in conjunction with a 533 MHz/256 MB DIMM.

More info here.

Nowadays, most motherboards support this feature. Two years ago, I bought my current motherboard and it has dual channel memory support. If you are buying a new one, it is always better to buy a motherboard with at least four slots and dual channel support (that is, if there are still those that don't support it).

As for arranging the use of memory slots, you usually have to put all the same modules in even or odd slots. For example, I have two 256DDR RAM Kingston modules and 4 slots on the motherboard. In order to function properly, I had to put these modules in slots number one and three. Later on, I will probably buy two same modules (which is recommended) and put them in slots number two and four.

Whereas we might not notice the difference when using dual channel memory and when not, it is still there, which can be proved by starting various memory tests - try that out and you'll see.

Yes it's an extension of the bank switching effect that makes these memories perform better. My dell laptop requires 2 memories in both slots, and they have to be the same size. I can't say it runs particularly fast but I'm not a gamer or anything so I really don't know.

Bought 2 1 gigabyte ram modules today. Well, I reckon it'll be enough. I doubt I'd experience a notable difference when using 4 modules (other than the price, which would be even higher).

They're PC 4200 DDR2 ram, Valueram from Kingston. They're supposed to be set on a motherboard I'll be receiving later, a P5B deluxe if I recall. Edit:
HV1125UKDE ASUS P5B Deluxe Wifi, Sock.775 Intel P965, ATX

Checked everything, should be all compatible. If something aint correct size I'll go cut it off

yup, 2 RAMs are of course better than 1, regardless if you have that dual channeling. However, I need more RAM myself, I can only get the PC2100 (266MHz) because my computer is just that old.

xboxrulz

One commonly overlooked (but rare) benefit of two sticks:
If one goes out, you can still run the computer off the other until you can get a replacement (assuming both are large enough).

~Viz

Well, for my motherboard, the slots are actually color coded. 1 and 3 is colored the same while 2 and 4 is of another color. Just another way to make sure which slots are paired together.

There are actually paired RAM modules certified to work in dual channel way. This dual channel thing is kinda fuzzy/complicated so if you want to get the maximum out of this dual channel thing, get those certified RAM modules.

It is true that with more than one memory stick, dual memory access han improve performace a little...

BUT, you are forgetting about the DRAM controler.
With High Speed DDR Ram, The Dram controler can run at a higher frequancy, and use a lower CAS latency when it does not need to control more than one memory module..

So it depends which is greater, the performance increace with Dual memory access... OR the perforamce hit the DRAM cotnroler takes having to use a lower CAS latancy.

ALSO...
1 X 1024Meg stick is often cheaper that 2 X 512Meg Sticks.