GPU question

It depends on the monitor resolution, the types of games you play, what your motherboard and power supply can handle, and what model of video card you intend to pay for.

The Nvidia GTX models have more flexibility in which cards you can pair, but generally the performance is pretty poor for what it costs.

The RTX line has a new pairing tech called NVLink which performs very well in benchmarks, but may be less great depending on the game. However, you can't link below the RTX 2080 model, and they charge extra for the NVLink bridge, so you're looking at around $1700+ or so for the entry level. Then, last I looked, the only 4k monitors capable of over 60fps cost $1600-$1800.

That's more to go on.

For 2x GTX 1070s your power supply is marginal at best, and depending on what all else is drawing power, may not be up to the task. You can check out your whole system at this website:

https://outervision.com/power-supply-calculator

I doubt the power supply can handle the load. You never want to be too near the limit of your PSU's output, or you'll burn it out too fast. If that's the case, you'd have to add the cost of a better PSU to the $300 or so for another 1070. Then you'd have to install it and re-wire everything. Then you'd have the issue of many games not even coded to recognize two cards, and most games being unable to give you much more than a 35% boost even if they do.

That said, it would give you an fps boost, just not anywhere near what you might expect.

An alternative might be to spend maybe $100 more for an RTX 2070. You wouldn't need a new power supply because 600W is fine. (But 600W will not power the 2080 or 2080 Ti just FYI.) Then you might be able to get some of that back selling your 1070.

The 2070 is considered the single card price/performance choice for solid frames at 1440p. If your monitor happens to be 1080p (because Dell makes both kinds), that card would kill everything you play. It would perform consistently well across all the games you play, instead of giving you uneven performance. I don't have a solid preference, just stating the facts.