If you are going to get a 4670k get a Z87 motherboard, should be cheaper.
Z97 are for the haswell refresh chips (4690/k & 4790/k; which should run cooler and OC better).
R9 280X is the 7970 from last gen, clock for clock mostly.
GTX 770 is an OCed 680 from last gen.
They run pretty much the same, except the 280X sucks when you have more than 1 of them.
Out of the two, I'd go for a 770.
If your budget can stretch though, the sapphire tri-x R9 290 is worth a look :p.
Try and get an ASUS or EVGA card, they tend to have better warranties and coolers. (Gigabyte is also solid, MSI is better than stock, but cheapo).
ASUS DCU II is one the best cards imo (depending on price); tends to OC like a beast and runs very cool. Downside is it doesn't blow out the back so much, just disperses it. With that current case and enough fans, I wouldn't worry though.
Also from what I've gathered, the 800 series is going to be focused on being more efficient, I wouldn't expect >10-15% ontop of the current range. (Considering they are stuck at 28nm for now)
Unless you plan on gaming at 1440p or above, I wouldn't worry about those cards too much.