How do I buy a graphics card?

I want to buy a graphics card.
As there are variety of cards available, I dont know which one is better and how that one is better.

Should I be looking at its memory size? (1 GB, 512 MB etc)
Or should I check its technology and ignore its memory? (GT, XT, etc.)
The RAM? (DDR2, DDR3)
The price?

Is a more expensive graphics cards always better?

What factors (performance, graphics etc.) can I use to determine which video card is better?

My approach, in light of the plethora of information contained in the spec of any graphics card, is to go by benchmarks. Specifically, Tom's hardware's benchmarks.

Of course, benchmarks should always to be taken with a grain of salt, since the benchmark rig might (and usually does) overpower your own in many aspects. However, I do believe that this is a good way to get a look at the "raw power" you can expect to get out of any specific card.

Tom's hardware also publish periodic recommendations to help you find the best card for your budget.

One final note: You should always check which DirectX technology your card supports (DirectX 10 being a requirement on some newer games, though almost any card on the market should support it).

I apologize for the link bombardment, but this is a really broad subject and there's always research to be done before buying a card.

Another option if best bang for your buck is what your interested in is http://www.pcpartcharts.com/filterGpus.php. This allows you to rank graphics cards by performance, by price and by cost-effectiveness for those of us with a budget.

A final reccomendation: It's really easy to get carried away and spent lots of money on a card. Usually that's counterproductive.

I found that it's almost always better to get the cheapest card that will get the job done and upgrade relatively frequently (1 year?), rather that get a monstrously expensive card planning to keep it for 2-3 years.

With this approach you will always have a card that is up to date, and it's often cheaper too in the long run.