The i7's main benefit is Hyper-Threading. The vast majority of games will not utilize the 8 threads you get, but most will make good use of four threads (all with a whole physical core with the i5, since it's quad-core).
Games tend to benefit most from Intel's single-threaded performance advantage, which doesn't change much between i5 and i7, though the i7-4770K has 8 MB of L3 cache over the i5-4670K's 6 MB of L3 cache, which may make a SLIGHT difference in the i7's favor per clock. Of course, that performance advantage carries across every thread, so games that can make use of the i5's four threads really feel the boost. The i7's Hyper-Threading isn't a complete linear scaling since it's not an octo-core, but benchmarks have shown it to be a lot more than expected. The thing is, most of those benchmarks aren't for games, but video encoding apps and stuff like that.
This may change in the future; people are citing Battlefield 4 performance as a possible sign of things to come, though I have yet to see any proper charts and graphs that a serious evaluation would show.
US$1500 is a great budget to work with; you've got plenty of room there for good parts! However, the fact that you're wanting to include a new monitor in that budget will eat into your funds. The monitors I'd want are often north of $250, and the more you spend on your monitor, the less you have to spend on what you're connecting to it.
For instance, the monitor I'd recommend for someone who prioritizes gaming above all else and needs something readily available (read: not a Sony GDM-FW900 that's been discontinued for a decade) is the
Asus VG248QE due to its high refresh rate and astoundingly low input lag, but that's $280 on Newegg and Amazon already, not including the price of shipping or the DIY NVIDIA G-SYNC kits that they're about to sell for it.
As for sound cards, I'm always going to favor the Sound Blaster X-Fi Titanium HD (partly out of preference for CMSS-3D Headphone, still playing older games and making use of some of its more exotic features), but it cannot be denied that the Xonar Essence STX and Sound Blaster Zx have gained a lot of ground with PC gamers lately.
Also, now's a fairly good time to buy since the sales are going and NVIDIA just slashed the price of the GTX 770 down to $320-330. That's right in the price-to-performance sweet spot at the time being. You should still have enough left over for an i5-4670K and a good Z87 board, especially if you live near a Micro Center store. (Their CPU/mobo combo deals beat out everyone else's by quite a bit, even factoring in local sales tax.)
