Now you've just got to get stacking down, which isn't hard, and you've mastered the white fluff.
Actually, you're pretty much on the verge of my own skill right now, so there's not a whole lot left I can teach you.
But those are some pretty good looking clouds, I think.
Once you've got basic stacking down and looking good, try stacking up and to the side, extending beyond the "base", and you'll be getting incrementally closer to the fancy clouds from earlier. Making bigger circles full of little circles might not be a bad route to explore next either.
Also, here's an example of the kind of clouds I tend to make:
Edit: Err... let's try that again with a background color, heh...
Here we go:
So you can tell me if you want any more advice or not.
Also, this is still just FLUFFY clouds - Mine are cumulus, yours up there is more like stratocumulus. The difference is that cumulus clouds are puffier and piled higher, while the stratocumulus are elongated and not as tall.
Stratus clouds have a completely different approach to them, although nimbus clouds are in many ways just super cumulus types with more complicated stacking (the fancy picture linked was of that type, I'd say) I have trouble with vertical cloud structures in general, though.
Ice clouds (high level Cirrus clouds) tend to be even more different, super wispy. Closer to stratus, I guess.
Stratus clouds can be a lot of fun to draw, and are pretty easy with a brush but not so much with pixels. I've never quite got the hang of getting them to look good, anyway.