A couple things, based on your questions.
Just to be clear, the 5600g is nothing but a stopgap. You can play SOME games at low settings, or more games if you down resolutions to 720p. At some point you may stumble on a GPU which you can toss in, and the 5600g will start performing like a 3700x, which isn't terrible. You should definitely read this to see what it can play and what it cannot.
The Ryzen 5 5600G is about to hit the retail market as the most affordable part sporting the Zen 3 architecture for $259. Inside the little box...
www.techspot.com
The 5600g will slot into any B450 or B550. I went with a budget board above, but you can pick from just about aby 450 or 550. BIOS flashback is probably important unless you want to borrow a processor from AMD to update BIOS. When you find a board, just make sure the 5600g is a supported processor. It likely is, but just check to be sure on the manufacturers website in the CPU support list.
List of B550 Motherboards With BIOS Flashback (AMD)
- ASRock B550 Taichi (ATX)
- Asus TUF B550M-Plus WiFi (mATX)
- Asus ROG Strix B550-I (mITX)
- Asus ROG Strix B550-A (ATX)
- Asus ROG Strix B550-E (ATX)
- Asus ROG Strix B550-F (ATX)
- Asus ROG Strix B550-F WiFi (ATX)
- Asus ROG Strix B550-XE WiFi (ATX)
- Asus TUF Gaming B550-Plus (ATX)
- Asus TUF Gaming B550-Plus WiFi (ATX)
- Gigabyte B550M Aorus Elite (mATX)
- Gigabyte B550M Aorus Pro (mATX)
- Gigabyte B550M Aorus Pro-P (mATX)
- Gigabyte B550M DS3H (mATX)
- Gigabyte B550M Gaming (mATX)
- Gigabyte B550M H (mATX)
- Gigabyte B550M S2H (mATX)
- Gigabyte B550I Aorus Pro AX (mITX)
- Gigabyte B550 Aorus Elite (ATX)
- Gigabyte B550 Aorus Master (ATX)
- Gigabyte B550 Aorus Pro (ATX)
- Gigabyte B550 Aorus Pro AC (ATX)
- Gigabyte B550 Aorus Pro AX (ATX)
- Gigabyte B550 Gaming X (ATX)
- Gigabyte B550 Vision D (ATX)
- MSI MAG B550M Bazooka (mATX)
- MSI MAG B550M Pro VDH WiFi (mATX)
- MSI MAG B550M Mortar (mATX)
- MSI MAG B550M Mortar WiFi (mATX)
- MSI MPG B550I Gaming Edge WiFi (mITX)
- MSI MPG B550I Gaming Edge MAX WiFi (mITX)
- MSI MEG B550 Unify (ATX)
- MSI MEG B550 Unify X (ATX)
- MSI MPG B550 Gaming Edge (ATX)
- MSI MPG B550 Gaming Plus (ATX)
- MSI MPG B550 Gaming Pro Carbon WiFi (ATX)
- MSI MPG B550 Tomahawk (ATX)
- MSI B550-A Pro (ATX)
List of B450 Motherboards With BIOS Flashback (AMD)
- Asus TUF Gaming B450M-Plus II (mATX)
- Asus TUF Gaming B450M-Pro S (mATX)
- Asus Prime B450M-A II (mATX)
- Asus ROG Strix B450-F Gaming II (ATX)
- MSI B450M Bazooka Plus (mATX)
- MSI B450M Gaming Plus (mATX)
- MSI B450M Mortar (mATX)
- MSI B450M Mortar MAX (mATX)
- MSI B450M Mortar Titanium (mATX)
- MSI B450-A Pro (ATX)
- MSI B450-A Pro MAX (ATX)
- MSI B450 Gaming Plus (ATX)
- MSI B450 Gaming Plus MAX (ATX)
- MSI B450 Gaming Pro Carbon AC (ATX)
- MSI B450 Gaming Pro Carbon AC MAX (ATX)
- MSI B450 Tomahawk (ATX)
- MSI B450 Tomahawk MAX (ATX)
Mobo size can be anything you want it to be, as long as it fits in your case. Case is purely an aesthetic choice. Just make sure it isn't an oven and has some mesh. I like the Phanteks P series as a generally good budget option with great air flow and fans included.
5600g comes with a cooler. It will be noisy, but it will work fine.
For RAM, 2 x 8 @ 3200 or 3600, CAS 16 is ideal. Brand usually doesn't matter, just pick something you have heard of (G.Skill, Team Group, Corsair, Crucial, etc.)