I don't think a SSD will help much, I have system monitors in a second screen and the drive monitor is completely flat whilst playing DF, it's all loaded into RAM, which by the way isn't used much either. The only time more RAM might be of use is during huge world gens and 12x12 embarks but remember to use anything over 2 gig you need to patch the .exe to be large address aware. However, I found if anything that just makes the game more unstable with normal use.
I was looking at overclockers to spec up a PC that would be a DF monster (but also had to be my main PC for about 3 years!) and picked out an overclocked 5GHz i7 3820 because the CPU cache was better then a sandybridge and so was the memory bandwidth. They only seem to go up to 4.6Ghz these days (stability issues?).
However, in the end I decided not to part with my cash and started playing Masterwork mod and designing my forts with FPS in mind, like 3x3 embark and getting rid of useless stuff, walling off unused areas and using DFHack to do things like 'clean all' and 'clean owned'. Certainly a cheaper option!
Just got a PC nerd friend of mine into DF though and he reckons he can build a dedicated DF PC out of spare parts that will rival any modern PC. Going to use a 3.4GHz dual core running XP and maybe overclock it a bit, be interesting to see how he does. Been a while since I've had a fort suffering from FPS death, my 8 year old fort still runs at 60fps most of the time on a 2.4GHz quad core. Just remember to set the processor affinity to realtime and stop watching internet TV in the 2nd monitor!