I Have No Real Idea What I Am Doing....

Hi, I'm new here, so please be kind.
I currently have an NVIDIA GeForce GT720 card. It is proving to be a bit laggy when I'm doing 4k video editing and definitely when I try to play the only game I do play- Eve.
I don't have an unlimited budget, but would like to upgrade to something that solves these problems. Any help and advice would be greatly welcomed. Oh, and based in the UK, if that makes a difference.
PC specs are:
CPU
Intel Core i7 4790 @ 3.60GHz 27 °C
Haswell 22nm Technology
RAM
16.0GB Dual-Channel DDR3 @ 798MHz (11-11-11-28)
Motherboard
Dell Inc. 088DT1 (CPU 1) 30 °C
Graphics
S22B150 (1920x1080@60Hz)
1023MB NVIDIA GeForce GT 720 (NVIDIA) 28 °C
Storage
894GB KINGSTON SA400S37960G (SATA-2 (SSD)) 22 °C
2794GB Samsung D3 Station USB Device (USB (SATA) (SSD)) 22 °C
 
It all comes down to your budget - how much do you want to spend as maximum? The next question to ask is whether or not the card will be used for gaming, other than just Eve Online. After that, we'll need the specifications of the power supply unit (PSU).
 
OK, my budget would be <$425.
The only other game I will be playing, assuming it is ever released, will be MS Flight Simulator 2020.
As for the PSU- I have no idea where to look for the information. Any help in this regard would be much appreciated.
 
Well, let's assume that the PC is an original Dell Inspiron 3847 (which is what the motherboard information suggests it is). Dell provide 2 good documents for this computer:



The first one tells you where the PSU is located and its exact specifications should be written on the unit. But from what I can tell, the PSU is a Dell-branded generic 300 or 350W one. It could actually be less than this, as GeForce GT 720 cards are only 20W, but the Dell documents state that the Inspiron 3847 could come equipped with up to a GT 705, which is 29W model.

To support modern, high powered graphics cards, you'll almost certainly need to upgrade the PSU. So some of that $424 will need to be set aside for this - something like the following will be enough:


However, it's important to ensure it will fit in the case, so open up yours and take some measurements of the unit's size. Let's assume that the above one fits, which would leave $374 for the graphics card and there's lots to choose from:


They're all good choices, but the best one for the money has to be the Radeon RX 590. However, it's about finding one that will inside the case - measure carefully how long the card can be before there's isn't room. Don't forget that big cards need their own power cable attached from the PSU (which is why it needs to be upgraded) so leave some space for that.
 
If you don't want to hassle with a PSU (does Dell still use a proprietary motherboard power connector?), you might want to just buy the very best video card you can get that does not need any 6 pin power connections. (eliminating the need for a new power supply) The rest of the system isn't bad for gaming, so coming from a GT720, almost anything is a good upgrade.

Something like a GTX1650 ($150) would surely make you happy. That's probably the easiest way to get more FPS out of a Dell or HP, (etc.)

This assumes it fits in the case. That card up there is relatively small but is dual slot. Upgrading an OEM machine is always an adventure. They weren't often built with upgrades in mind.
 
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