If I may offer a concise answer:
Because while today's TVs have a multitude of direct input options - RGB on phono jacks, SCART, HDMI and others - there used to be only an aerial socket. So devices like VCRs and video games had to pretend to be a TV program, broadcast on a specific channel. Obviously it was a good idea to choose a channel that was 'spare',...
Why tune the TV to a channel at all?
Early consumer television sets were built for one purpose: to watch broadcast television. The only input was for an antenna. The idea of using your television set for any other purpose simply wasn't yet conceived, so there were no connectors for any other kind of signal.
The first additional uses for household ...
The bandwidth of a typical composite video signal is about 6 MHz.
The video hardware in in early game consoles (and early home
computers) generates its composite video signal in what's called
baseband format: the frequency range is 0 Hz to 6 MHz. Only one
baseband signal can be sent down a cable or broadcast through the air
at any one time.¹
So that we can ...
In the US Channel 3 was preferred as it was in the midst of the continious 2/3/4 assignment. By lifting or lowering the carrier by 6 Mhz either channel 2 or 4 could be offered as alternative.
Another-Dave already answered the basic issue of avoiding collisions. The reason why it was channel 2/3/4 is due historic channel assignment (in the US, *1).
Because those are the two frequencies that the RF modulator in the game console can produce.
"Tuning to a channel" means "choosing the frequency band to receive", so clearly you need to receive on the same frequency band that the transmitter is sending.
(Warning, non-electronic-engineer loose wording above).
Why was there a choice of two rather than one ...
With only a 250W power supply you may be pushing the limit with two video cards, but it's hard to be sure. Neither of the video cards you're planning on using are particularly power hungry. I suspect nether comes with a fan, and the GeForce2 MX may not even have a heat sink. The PCI connector can only supply 25W and I believe the AGP 1.0 interface had the ...
That's tricky. PC video cards had to be compatible with old CGA/MGA/EGA/VGA standards over ISA which didn't offer plug and play and remapping.
Your two video cards may be incompatible, conflicting over accesses to registers, for example the palette or Video RAM in A0000.