ok, you have a dds called blah.dds, which you want to use. Make a tga out of it, ( blah.tga ) apply it in RenX, and use the dds as texture in the texture map. Done
DDS is a more highly compressed system and generals works better with DDS files because it can apply them directly and with TGA's it works different (engine thing).
Also DDS is smaller file sized, thus meaning faster rendering (engine thing)
So to make it easy its better performence in generals
dds is a better qwality of images better then tga when u zoom in tga file it gives a wierd color in the game if u export it as dds you see a better qwality of skin