I'm looking into building a new computer and I can't decide what graphics card. Instead of just being told "use this one" I want to know why. What should I look at when comparing cards? What numbers are important for gaming? Is it Shader count? Shader speed? what about memory speed? Whats important is what I want to know. I have one person saying that the new 660ti is just a bit better then the 570, but the article here on tom's hardware sounds like it's a good amount better then the 580. Help?
You can look at the specific specs of the cards, but the only thing that really matters is benchmarks. Case in point, the older 500 series GTX cards will have a lot less cores than the newer 600 series cards, but that's mostly because the cores of the older cards were clocked a lot higher than those of the 600 series cards. Now obviously the 600 series is more powerful, but a direct comparison of the number of cores is useless.
dudewitbow is right, unless the cards are in the same family, a direct comparison cannot be made.
Ultimately, what graphics card you choose is dependant on your budget, and what you will be using the card for. Assuming you will be gaming, and are looking for a card around the 660ti bracket -- I'd say wait a week or so and look out for the 7870 to drop in price. The 660ti is definitely a solid card but the 192-bit memory doesn't do justice to the 2GB of VRAM. The 7870 trades punches with the 660ti, losing by a 5~ fps on most games.
However, Pitcairn AMD cards have built up a solid reputation for being great over-clockers, so you could definitely OC the 7870 to stock 660ti speeds and possible even further, without the memory bottleneck. And at a 250$ or so price-point, let's say, the 7870 would definitely win out as the value choice.
When you climb up higher to 670/680/7970 territory, things become teeny bit more complicated, not too much however. I'd say post your intentions with your card, then we can make an assessment.