If you want landscape generators/makers then terragen is good (terragen 2 is out soon!)
Mojoworld is the best though IMO. I can't afford it, but you can get a 30 day trial for free.
I cant remember the URL sorry, so just google if interested
That's excellent - you've got the surface maps down pat there. Atmosphere's excellent, but I'd like to see you start playing with clouds, and some point - you can get some fantastic effects by changing the cloud settings and sky heights.
Im not sure as I dont have the program on this PC, but i think its in the landscape section. Add more children to the surface map and alter the colour and settings. If I'm not being clear then just google terragen surface maps
On your surface map click "add child". Edit it and make the child layer white, keeping the bumpiness on the high side but not too high. Then you can edit the scale (higher = deeper snow), distribution and slope. Just fiddle with those settings until it looks like you want; may take you half an hour or longer.