can we use 1gb or 2gb ram on any computer.  

Depends on what the age of the mother board is and what it supports. Most new mother boards are supporting DDR Generation 2 ram (second generation of DDR). If you have an older board most likely you are looking at SDRAM or synchronized RAM. There are several technologies of RAM out there.

The oldest being the 64, 128, 256 ram, then SDRAM, then DDR (1st generation) and DDR 2. In between the SDRAM and DDR there is a second generation SDRAM that supports PCI 100 and PCI 130 (speed of RAM processing or ability to cache memory separately from the mother board).

Not every computer can support all types of ram. Most asus boards (newer versions) can support one or the other of SDRAM or DDR\DDR 2.

In lay man's terms, not all mother boards support 1 gb of ram and it depends on how old your mother board (that big board that runs everything else, the one to the back of the computer) is. Most boards from the year 2003 to now support 1 gb of ram or more depending on make and model of board.

Talk to the vendor first. They will know whether your motherboard is upgradable. That means the motherboard has an extension lot for extra memory. Sounds like you have a 500M RAM at your current computer. If you do have an extension lot for memory, upgrading is easy. Just insert the memory to the slot.

First consult your motherboard manual for Memory type and capacity that your motherboard will support before you buy.

Its a bit difficult to tell that.
But it depends on your computer.
call any vendor who will put the RAM, show him your computer, he'll open the CPU and tell you whether you require a DDR1 or DDR2 RAM,
Then goto canirunit.com,
there select any game which has high requirements such as Crysis, then it will show you a result after some time, there it shows how much expandable your memory is..
Hope you upgrade soon!!

Gp to Geek squad inside of bestbuy. they will tell you everything you need to know.

Determine first what your motherboard is capable of using SIW. If its a DDR and you really are on a tight budget then I would suggest you earn some more. In our place the price of DDR is twice the price of DDR2. It would be better if you replace mobo+processor+RAM, the cheapest processor is definitely faster than your current one, plus you'll have more room for upgrade than your old one.

Best & easiest way to go is to look at your motherboard manual - everything is written there!~From the max. capacity RAM supported & what types. Well, if you've lost your manual, you can google your mobo model & you'll see all the specs you're looking for

It depends on how old the computer is what type of memory it uses. If its newer it probably uses DDR or DDR2 or 3. If its an older computer it uses SDRAM. You should look at the computer manufacturers website for the computer and you should be able to find out what it supports. Or you can go to a site like crucial.com and you can search by manufactuerer/motherboard/etc. That will tell you how much your computer is able to hold.