How to buy computer MEMORY (RAM)

I am using window vista. I have 1 GB RAM. But for fast computing I decide to add
More ram.
Yesterday, I went to a shop for buying computer memory. These are the some steps I follow for buying a better computer memory. These steps helped me so I am sharing these steps with you.

DDR1 OR DDR2 OR DDR3

DDR1 is history now. It is very costly and not available in many stores because it is out of production. If your motherboard supports only DDR1 memory then change your motherboard or buy a cheap 2nd hand memory.
DDR2 is your best bet. It is cheap and high performance than DDR1.DDR2 is available in mainly three clock speeds 667,800and 1066 MHz .For a home user even 667 MHz is sufficient.
DDR3 promises to become the new standard in this year and coming years.DDR3 is available in many clock speeds from 1333 to 1800 MHz’s. Intel has announced support for DDR3 as a memory platform due to the increased bandwidth it provides (eventually, DDR3 will scale up to 2 GHz).But DDR3 is very costly.

Conclusion
Enough money = if you have enough money then buy a DDR3 enabled motherboard and DDR3 memory.
Less money = if you have less money like me and wants to be intelligent customer then buy a DDR2 memory or wait for 6months for drop in the price of DDR3 memory and their motherboards.
And, most important don’t buy unbranded or locally-branded memory. It’s a very stressed-out component in your PC, and the number one cause of freeze-ups and restarts.
Invest wisely—a few hundred bucks more for peace of mind.