DXT3 and DXT1a are fine to use with FSO. DXT3 is more like DXT1, but with 4-bit alpha. If you don't need a lot of alpha gradients then DXT3 is a good way to go since it can produce a little better quality than DXT5 will give you. DXT5 has full 8-bit alpha which is a good format for anything needing alpha, unless you just know that DXT3 is all that you need. Most people just go with DXT5 when needed alpha though, and there is nothing wrong with that.
As far as the game is concerned, DXT1a and DXT1c are the same thing. Someday this will probably change, which is why I tell everyone to use the proper version for their non-alpha files. When I do a bmpman upgrade next year the two DXT formats may be handled properly and give you the 1-bit alpha if you wanted it with DXT1a. So unless you actually want alpha, be sure to use always use DXT1c.
The uncompressed formats which are supported are: u8888 (32-bit), u888 (24-bit), u1555 (16-bit), and p8c (8-bit). The 16-bit format is what PCX images are turned into when loaded by the game, even though that still only provide 256 colors. If you can get away with it, and don't need/want the compression, then the 16-bit DDS can be a good one to use since it will need only half the memory as the 32-bit format. The 8-bit DDS format isn't properly supported right now, though it will load the files. When time allows me to do so I'll go back and finish up support for that.
DXT2 and DXT4 are the two formats which you can't use with FSO, but then most DDS utilities can't make them either so it's not a big deal.
For an idea on memory usage for the various formats:For uncompressed formats, memory size is determined by the following formula: width*height*bit-depth. With "bit-depth" being 4 for 32-bit image, 3 for 24-bit images, 2 for 16-bit images, and 1 for 8-bit images. Which, for a 515x512 image, gives you something like this:
32-bit: 1meg
24-bit: 768k
16-bit: 512k
8-bit: 256k
Compressed image formats (excluding something like JPG, which uses the above calculations) will be almost exactly what their file sizes are. But, using the "bit-depth" reference for the above image types, it would break down like this for compressed types:
DXT5: 1 (~256k)
DXT3: 1 (~256k)
DXT1a: 0.7 (~179k)
DXT1c: 0.5 (~128k)
DDS also offers you mipmaps for every format though, and that will add extra to the memory needed (but 99.9% of the time it's better for game performance to have mipmaps over the memory they use). When you have a full set of mipmap layers (the default for everything) you will add approximately 33% (the basic high average, not totally accurate) to the filesize. So to get the final memory size of the above formats, just multiply the current sizes by 1.33.
Hopefully that answers most everything.
