GTX 770 and the R9-280x are probably the best GPU's for the best value. Either or, you can't go wrong. Ican confirm the GTX 770 to be a really good gpu. I've got some really good frames on most games and great OC, my core usually boosts about 150mhz more and memory about 300mhz more with a simple slide of the button.
This is personal preference but every time i've had Nvidia, i've always had good luck with EVGA, never had a single problem from any of theirs. On AMD, i've had tons of luck with MSI and ASUS, not so with Gigabyte, HIS, or XFX. Never tried Sapphire but they seem to be the prime card provider along with Powercolor.
I never understood buying CF or SLI unless its top end GPUs and you're running crazy amounts of monitors and it's necessary...
the 280x's are going for $400-470... I haven't even seen a $360 option lately... you can wait for prices to come down or go with a gtx 770 2GB which will run most games at ultra and the new ones at high settings... the 4gb model bottlenecks itself, so save yourself the money there unless you plan on SLI... and if you plan on SLI get the 780