Hi and welcome to Tom's Hardware! I'll go straight to your questions:
1) what would be the best bang for the buck combo (monitor + GPU) for a 1080p 144Hz setup?
A 1080p/144Hz monitor is recommended to be paired with a GTX 1080 (as the GTX 1070 is ideal for 1440p/60Hz or 1080p/~100Hz). Moreover, current prices of the GTX 1070 are so close to that of the GTX 1080 which makes the more powerful GPU a better value, despite a slightly higher cost.
If I were to get a 1080p/144Hz monitor, playing FPS (fast-paced) types of games, I would look into one that has G-Sync (variable refresh rates for smoother gameplay) when paired with an Nvidia GPU such as the aforementioned ones:
PCPartPicker part list /
Price breakdown by merchant
Video Card: MSI - GeForce GTX 1080 8GB DUKE OC Video Card ($514.98 @ Newegg)
Monitor: AOC - G2460PG 24.0" 1920x1080 144Hz Monitor ($339.99 @ Best Buy)
Total: $854.97
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-07-30 07:56 EDT-0400
2) If I have a budget of $1000 for both GPU and monitor, what would be the ideal gaming setup (ie. one that would not choke on my CPU/mobo)? should I go for a 1440p 144Hz monitor? should I go for GTX 1080 or GTX 1070 would suffice?
The CPU/system that you have is a very powerful one, that can even handle the most powerful GPUs around. So, whether you choose a GTX 1070 or GTX 1080 (or even a GTX 1080 Ti, if budget permits and current PSU allows), you won't have to worry about hindering the GPUs maximum possible performance using an i7-4790K CPU.
As mentioned above, a 1080p/144Hz monitor is recommended to be paired with a GTX 1080. That is not to say that a GTX 1070 will struggle in all games. If you take into consideration specific non GPU-intensive games, such as CS:GO, even a less powerful GTX 1060 can achieve 144fps on maxed settings in 1080p resolutions. However, other more GPU-intensive games (such as Overwatch), the GTX 1080 is recommended, much more for AAA games (like GTA V, Fallout 4, and the likes, which you
may still have to decrease some graphics settings in order to get ~144fps on a GTX 1080 in 1080p).
So, with a 1440p/144Hz (higher pixels than the 1080p/144Hz), the GTX 1080
may struggle on AAA games at Ultra graphics settings (but definitely not on CS:GO and similar games), to which, most recommend going for a GTX 1080 Ti (a 4K/60Hz GPU). But if you are amenable to decrease in-game graphics on certain GPU-intensive games (or if you mostly play the likes of CS:GO), then the GTX 1080 will still suffice, especially if your budget cannot exceed $1000. The 1440p does produce more details compared to 1080p. The higher resolution will also be better for the occassional photo/video edits you will do.
To give you an idea of the cost for a 1440p/144Hz (with G-Sync) + a GTX 1080 (fits your $1000 budget):
PCPartPicker part list /
Price breakdown by merchant
Video Card: MSI - GeForce GTX 1080 8GB DUKE OC Video Card ($514.98 @ Newegg)
Monitor: Dell - S2417DG 23.8" 2560x1440 165Hz Monitor ($399.99 @ Amazon)
Total: $914.97
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-07-30 08:05 EDT-0400
To give you an idea of the cost for a 1440p/144Hz (with G-Sync) + a GTX 1080 Ti (would exceed $120 over your budget):
PCPartPicker part list /
Price breakdown by merchant
Video Card: EVGA - GeForce GTX 1080 Ti 11GB SC Black Edition Video Card ($719.89 @ OutletPC)
Monitor: Dell - S2417DG 23.8" 2560x1440 165Hz Monitor ($399.99 @ Amazon)
Total: $1119.88
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-07-30 08:06 EDT-0400
Hope these thoughts would help you out in a way.
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