Gigabyte cards are a bit cheaper for some reason, maybe just made a little cheaper, or possibly just offer better prices. I always like EVGA and Sapphire cards, but the graphics processor unit (GPU) is always either Nvidia or AMD. Some cards have better cooling styles or use a quieter fan, more unique design, etc.
Things to look at in a video card: (keep in mind that all the cards you are looking at are really nice)
AMD/Radeon: 5000 series is newer/supposedly better than the 4000 series.
Nvidia: GTX 295's are better/faster than the GTX260. Before all that was GTS 250 and the Geforce series, like a 9600GT being better than a 8600GT. (All generally speaking of course, in a nut shell)
Memory Clock: The higher the mhz the better.
Core Clock: The higher the mhz the better.
Memory Size: Higher is better.
Memory Interface: (bits) Again, higher is better
Memory Type: GDDR5 is better than GDDR3
DirectX: Higher is better! Do you noticed a trend here? DirectX 11 being the latest and only compatible with Windows 7, not sure about Vista.
OpenGL: Higher is better, and I don't know much more than that on this one.
Max Resolution: Always pick a card that supports the minumum resolution that your monitor supports and/or that you plan to play your games at. Most modern cards support well over High Definition... 1920 x 1080p.
Display Ports: The 5000 series of Radeons actually support up to 3 monitors, usually with 2 DVI ports, an HDMI port and what always has to be used when using the 3 monitors the Display Port. (Eyefinity
is what they call it) On most modern cards, there will be always two DVI ports or a DVI and a VGA which means it will support dual monitor setups
P.S. Most cards come with at least 1 DVI to VGA adapter because many monitors still use VGA connections.
Power Connectors: This is pretty important. You MUST purchase a power supply that meets or exceeds the video cards power requirements. Furthermore, that power supply must have the appropriate and/or enough power connectors(PCI-E connectors) to plug into the card because these high end cards require there own power source. Most of the cards you are looking at require 2 x 6 pin power connectors, which means you will be plugging in two connectors usually located at the front of the card. Some power supplies will have something like a 6 + 2 pin connector which can be used as a 6pin or combined as a 8pin. Look at these here... 6pin and a 6 + 2 pin
. They would plug into something like this on a card... CLICK HERE.
Ok, I know that may be a lot of information, but you asked for it, so there you go. You can always refer back to this topic and see that if you ever need to, they keep topics around forever here.
If you really want a point in the right direction, go with a 5770, that's really new technology, supports DirectX 11, look for a nice core clock and memory clock, and the higher the "bits" the better. Also has support for 3 monitors if you really want to do something fancy in the future.
Keep in touch!