Dual-channel memory

Quad-channel vs dual-channel is simply the way your PC's memory controller is natively configured as. Any DDR4 RAM that's unbuffered and non-ECC may be used in any PC. Buying a quad-channel memory kit simply ensures that all four sticks of RAM are compatible with one another in the same PC. Buying two dual-channel memory kits, on the other hand, runs the risk of having the sticks from one kit being completely different (IC-wise, and therefore characteristics) than those from the other - and in many cases, you will not be able to run both of those kits in the same PC at their advertised speed. In a few rare cases, the two kits might not run together well at all in that particular PC.

As for your plan to replace the Quadro K5200, now is the time to do it, especially with today's GPU pricing that's close to (but still somewhat above) where they should have been in the first place. You see, all driver support for all Kepler Quadro desktop GPUs will completely end this coming January 31 (as in January 31, 2023). But with that i9-10940X that's now three years old at this point, it would be better not to waste $800-ish on that new RTX 3080 because that CPU is now outdated, using a derivative of an architecture that dates back to the first Skylake-X cpus released in mid-2017. And with that CPU actually performing worse than a cheaper 12-core/24-thread AMD Ryzen 9 5900X, you can actually save money by choosing an RTX 3070 or even an RTX 3060 Ti instead, and not lose much if anything in terms of overall performance. Go with an RTX 3080 with that 10940X only if every single video project includes at least some GPU-accelerated effects, and/or if you constantly downscale everything dramatically (say, from 4k all the way down to standard definition).

You must also install according to your motherboard manual

My ASUS Z490-P motherboard has 4 ram slots A1 A2 B1 B2

1 ram stick goes in A2

2 ram sticks go in A2 + B2

And 4 sticks go in all 4 slots

Ram sticks MUST be the same size and timing/speed specification to work properly

I've moved this from the Using the Community forum (which is the forum for issues using the forums) to the Video Hardware forum so that proper help can be offered.