Upgrading a video card if possible in my X51.

Hi folks,

I have an X51 that I purchased in 2014 that has the following specs:

i5 Core Processor at 3.0 GHZ, 16 GIG RAM, and a GeForce GT 640 video card with 1 GIG of RAM.

I am trying to play Sea of Thieves and meet the minimum game values, however the minimum video card is exactly what I have, being 1 GIG of RAM so I'm experiencing very low FPS, sometimes around 4, but never above 20 and it gets super choppy even running the game with all the settings on "Cursed" which is the lowest value and the Pixel limit on 780 which is also very low for the game.

I went to BestBuy to grab a new video card and the lowest end they had was a RADEON RX 560 with 4 GIG of video memory and I tried it out, but not being great with PC building and what not after installing it and experiencing 50 FPS and great gaming for about 2 minutes my system shut down from over heating. The card said it required a 400 Watt power supply and I believe mine is at 240 Watts which came with the system. (Input 100-240V~50-60Hz 3.5A) listed (Output 19.5V~~~12.3A) which volts times amps equals watts I believe and it's right around 239.x so right at 240 which I believe is the limit.

I returned the card and the Geek squad people there told me that's the lowest end card they have for my system and that for this PC upgrading it is not really an option being very compact. I checked there "Gaming" PC's out at the store and the best they had was 3.0 GHZ with 8 GIG RAM and a 4 GIG card which is barely better or nearly equal to my 4 almost 5 year old PC. So I'm wondering if anyone has had my system setup and found or knows of a video card that's compatible with it.

Thanks in advance.