Budget PC build for 20 man multibox.

[MiRai]

Is base clock on CPU that important even if it goes to 4Ghz in turbo boost mode?

"Boost" is nothing more than marketing BS because it normally only affects one or two cores, and if you're cooling on air, then it's going to be for a limited time (unless you adjust settings in the BIOS and are willing to run at a higher temperature).

EDIT1: There's a reason why there's a 1.2GHz gap between the base clock and the turbo clock. If the turbo clock was easily sustained then the gap wouldn't be so large (it's almost 50% of the base clock of the CPU). Also, 2.8GHz isn't the base clock for "power saving," because that's what C-States are for.

EDIT2: GamersNexus has an article on this that shows the boost tables for CoffeeLake CPUs, including the 8400 (their table is wrong, it's 4GHz and he states that in the associated video), and here's a thread over on Tom's Hardware where TechyInAZ explains it. In so many words, the maximum boost clock is only achieved when you're using one core, and then it drops down from there based on the amount of cores that are being used, which is then based upon thermals.

Is 8400 worser than 7700k for multiboxing?

Slightly, yes.