That's not an 8088 machine (with 8b external bus), but an 8086-class one, with 16b external bus, and V30 CPU. Much more powerful beast, although in terms of instruction sets it's basically still and XT.
Looking at the pics in your own link, it has 30p SIMM slots, so you upgrade the RAM with 30p SIMMs. The V30 still has the 8086/8088 limitation of a 20b address bus, so max directly accessible RAM is 1MB, and only the first 640kB of that is available for programs. So 2x 256kB is the only sensible option.
If you want more than 640kB, you need to use EMS, so you need to look for either a vintage EMS card, or a modern implementation such as the Lo-Tech 2MB EMS card. Note that your software also needs to support EMS to use it.
Edit: looks like the board is more advanced than I credited it with. 2x 1MB SIMMs are the way to go then.
As for VGA - it could be possible, maybe you don't even need to turn it off. What do you think to gain by an external card?