Originally Posted by Offler
My current build (still in progress) in sub... I would go for AM4 socket, probably an AsRock or Asus MB, something in third gen Ryzen. Still Gskill FlareX, because those have Samsung B-die.
Case would be some mITX, with capacity to house one larger GPU.
The AsRock BIOS is pretty annoying to work with. It has a lot of features but a decent chunk of them are broken (maybe it's better now). VRMs on the ASUS B450 or X470 boards are pretty bad until the very high end, though at that point you're better off considering some of the $150-$200 X570 boards Buildzoid recommended in his motherboard roundup instead for better VRMs at a lower cost.
$130 would be the most I'd spend for non-X570 which would be the MSi B450 Gaming Pro Carbon AC. Buildzoid is slightly on the fence with that board just because a very good X570 board is right at $130 (Asus Prime X570-P) with another at $165 (ASUS TUF Gaming X570-PLUS), but if you don't plan on overclocking with a 3900X (stock 3900X or overclocked 3800X and below are fine on the good B450 boards) and either 1. don't want the chipset fan, 2. want Intel LAN, or 3. were going to get Wi-Fi anyways, this is a good board.
The B-die doesn't mean much if he isn't going to spend the time trying to tighten up his secondary timings and there's bad B-die as well. A CJR kit will be fine so he can at least get an FCLK of 1800MHz with no extra work. E-die if he wants to save another $30 over the CJR kit he currently has and is willing to spend the minimal effort needed to use the Ryzen DRAM calculator and copy paste those timings into his BIOS.
Edit: If you plan on overclocking with the 3900X, I'd swap to the ASUS Prime X570-P at the current $127. The B450 Tomahawk MAX and Gaming Pro Carbon AC are fine with the 3700X/3800X but a 3900X is pushing it if you're not running stock clocks (I edited my post on the 1st page to change the conflicting info there). Next meaningful upgrade would probably be the ASUS TUF Gaming X570-PLUS at $165. Get the TUF if the Prime X570-P goes back up to the normal $150.