The letters in the lower corner represent the sound that the material will make in game.
M for Metal, material ID 2
S for Stone, Material ID 1
W for wood, Material ID 4
Your custom textures should be 1024 x 1024 and be tile able.
Put them into the Game Guru\Files\ebebank\texturesource folder. Diffuse, Normal and Specular maps needed for full shader effects. _D, _N, _S. The _S texture has an alpha channel in it for some reason that I am not sure why yet. Save this one at DXT 5 format. All textures used in the Beta so far have been .dds format. Not sure about .tga and .png files working proper yet.
The maps you choose in the EBE editor will be combined into a 4096 x 4096 sheet and the normals and spec will be auto generated to make the sheet. Sheet names can be changed I believe for you to have different ones for later use.
For your last question I will have to play with it to see if I can get an answer for you.
Intel(R) Core(TM)2 Quad CPU Q6600 @ 2.40GHz, 2400 Mhz, 4 Core(s), 4 Logical Processor(s), 8gb RAM, Nvidia gtx660, Windows 7 Pro 64bit, Screen resolution 1680 x 1050.