I'm assuming it's an original 800 and not the XL?
If so, get yourself a decent well shielded S-Video cable, I'd recommend the one from Hercules....
https://www.herculesworkshop.net/atari and commodore video cables.html
And then I'd recommend the Retrotink 2x, you may be able to find t cheaper on eBay or elsewhere but
this is the official site....
https://www.retrotink.com/product-page/retrotink-2x-pro
It scales beautifully, has several options for the ratio and scaling and has worked with every monitor and TV I've thrown it at.
I've been through soooooo many crap HDMI converters on Amazon, in the long run I'd really recommend avoiding the headaches
and grabbing the 2x.
You might also consider the SIO2SD or SDrive Max, maybe the fujinet. Unless you're a purist and want everything on floppy, these inexpensive USB alternatives give you access
to the entire library in one shot, every game, app, the whole enchilada.
Anyhow, enjoy the 800, it's an absolutely wonderful machine and a piece of history!