There are several texture quality issues in your picture.
You’re obviously rendering with MSAA (or SSAA) so that explains why your geometry edge quality doesn’t have the jaggies. Remember, with MSAA, only one texture sample is taken per pixel (not per sample), so the texture sampling rate is lower than your geometry edges. However, texture accesses are filtered (if you set up the texture right), approximating a higher frequency sampling rate. But this approximation is poor at more glancing angles, unless you crank up anisotropy.
What it looks like on the planet up-close is you’re magnifying the texture. And there are ugly ringing-like artifacts around the text. You didn’t swipe this texture from a JPEG, did you? That might explain it.
And farther toward the horizon, it looks like you only have a low-res texture paged in. I’d say you just needed to crank up aniso, but there is a nice crisp line where it goes from clear to blurry which nixes that as the only cause here. You can try cranking up aniso on the texture:
glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MAX_ANISOTROPY_EXT, 4 ); // or 8
but I don’t think that’s gonna get rid of that crisp/blurry boundary. You do have MIPmaps defined on this texture and have set MIN_FILTER to LINEAR_MIPMAP_LINEAR, right?
Now as to your decals, yes, unless you want a flickery mess, you’re going to want MIPmaps defined and a LINEAR_MIPMAP_LINEAR MIN_FILTER enabled. Face-on this will look OK and not flicker, but as you get edge-on, it’ll blur out too much (screen X and Y derivatives become widely different, so it pushes your sampling up to higher (courser) MIP levels. To combat, tell the GPU to take multiple samples at lower (finer) MIP levels and average them for higher quality. You do this by cranking up anisotropy on the texture.
For testing purposes though, you can tell the NVidia driver to render with SSAA. That’ll sample the texture multiple times in the pixel and avg the results. Here in Linux, looks like there’s one lone mode holding out with SSAA support; mode 9:
9 - 8x (4xSS, 2xMS)
What GPU are you running on? If pre-GeForce 8, recall that GeForce 8 “vastly” improved texture sampling quality, which would help with the clarity of your decal and other textures, especially when viewed at a diagonal angle like that or edge-on with aniso cranked up: