Is there any particular specs you would be looking for in a gpu? For example, you mention that the card with more ram had a narrow memory bus. Which spec on a card would be most important, etc.?
There are a few factors you want to look for in a really good mining GPU, in no particular order.
- Stream Processors. This is essentially how many "cores" your GPU will have.
A 7770 will have 640 processors. A 290x has 2,816, over 4x as much.
- Memory. Scrypt is very memory intensive, so higher speed, and also higher bus width are important.
A 7770 has a 128-bit width, and runs at 1125MHz (real), 4500MHz (effective). A 280x has a 384-bit width (3x as much data can be moved through the RAM), and runs at 1500MHz (real), 6000MHz (effective), so another ~33% faster.
- Core speed. Less important, as this will need to be tweaked anyways to get the best performance. It is much easier to downclock a factory OC'd GPU than it is to push a lower speed chip to it's max, tho.
- Voltage unlocked. This is hard to find out, as it's not something advertisers will promote. But, it can make a huge difference when running lots of GPUs. You have to check for each specific card you're looking at buying.
- Cooler. You will need the best cooler possible to prevent destroying your GPU. Lower end cards can get away with only one fan, but for high end cards you will want 2, if not 3 fans. This is assuming you're not getting a reference blower cooler, which is one fan at the back of the card. I like those, but they are loud...
Those are just some general guidelines, and once you start looking into the dozens of GPUs available, you'll start to figure out patters in which brands mine the best, which specific cards are really great (and which ones aren't), and which ones are easier to cool.
