Also remember that DDS images don't have to be compressed. You can have uncompressed 16, 24 or 32-bit DDS images that look the same as TGA versions. They take up the same amount of memory as the TGA version too, but they don't require -jpgtga to be used. My recent changes allow for uncompressed DDS images to be used even if your card can't support compressed DDS. Also a 16-bit DDS would end up using the same amount of memory as a PCX image but has the advantage of using the full 16-bit color space rather than having to deal with the crappy 8-bit to 16-bit color conversion that PCX has to go through. So if you need more color than a 256 PCX gives you, but not as much as a 24-bit image and compression doesn't work well, then go with a RGB5_A1 DDS image instead. You can get the needed format by using nvdxt with the -u1555 option (and maybe the -swap option to convert to BGR order). In nvdxt the -u8888 option produces the same result as a 32-bit TGA, the -u888 option produces the same result as a 24-bt TGA (no alpha).
As DaBrain said, DXT1 has the least memory usage but doesn't have alpha (for speed and other reasons the OGL code has been modified to use color DXT1 only, no alpha bit). DXT5 offers 8-bit alpha just like 32-bit TGA does but since it has a lower bit depth per block it can introduce banding on gradients. So if you are converting images which need an alpha component use DXT5, otherwise use DXT1c (the color only version). If the compression artifacts are too bad then try for the uncompressed DDS versions mentioned above and go for 16-bit DDS if at all possible.