I'd agree that a SSD is probably more of an improvment instead of going from from the 920 to the 950. You can always OC the 920 a bit if you want to.

12GB of RAM is probably more important than either the 950 or the SSD. For most things in northrend, and especially Dalaran and WG you will need as much memory as you can get. If you absolutely need to cut costs, then maybe get 3x 2GB sticks initially, just plan on getting 3x2GB more sticks before you level to northrend. (If you're doing multiple RAF accounts to 60 you may have a few months before you get to northrend.)

Concure on the SSD as well. The "cheap" option here is a 32GB SSD ($120) that's just for WOW and a seperate standard HD ($60-90 for 500GB-1TB) for everything else. You can easily symlink 5x WoW folders there and still be under 17GB. That leaves room for some other apps/games, but not a whole lot. Good 64GB drives can be had for $200 and good 120GB drives are under $300 now. How much stuff you want to put on it is really up to you. But you can get blazing fast performance + room for storage for under $200 if you want to.