Well, since we are guessing, lets think of something Interesting
Perhaps:
R300 = ATI X
I like that name cause it is the Roman Numeral for 10, which would indicicate kinda Radeon 10000 and also would elude to the ArtX technology in the R300. Kinda catchy.
NV30 = I don't know about Eclipse, I kinda like it, but it reminds me of the car, the Mitsibishi Eclipse which is not a good car IMO. So I dunno what I'd name the NV30 right now, something that would kinda elude to 3dfx would be nice.