If all the "noise" you're referring to is coming from external displacement/texture maps, then the issue is probably in the maps, it's not so much a rendering issue. Use a 16bit/channel data format (16 bit TIFF or EXR) for best quality, and antialias your text. Or, if you're just using the text as a test (i.e. you don't actually want to render letters on terrain, but thought it would be a good way to get a clear reference), then I suggest actually trying your real data, whether it's a hand-painted displacement or mask, or whatever. Extremely hard edges like on type are relatively unnatural and you probably won't see such problems with a more real-world terrain.
- Oshyan