I am very new to the computer scene and i was wondering when looking at different gpu's what is the most important think to look for, what is the least? also can someone describe what stream processors are?
Getting a video card is a balance between your budget, expected performance, how you intend to use it, what resolution, and what your current system is. I am assuming you are refering to games. Each card handles certain games better than others. It's hard to say "That one is better".
Right now the 4850 is the best price to performance card. Meaning it gives you the best bang for your buck. Is it the Best? NO. But for $150 you won't get a card that performs any better. For $50 more you can get the 4870, for $50 more you can get the 4870 1GB version, for $500 you can get the 4870x2. There there is Nvidia's offerings.... get the drift.
You can also let us know what you are looking for, are you trying to research a card to buy ?
If you post your system specs including Power supply and Monitor size/resolution as well as CPU,RAM and Operating system we could offer sugestions on suitable cards and reasons why.