Obviously if you're working with massive datasets or many many VM's this may not be true, but everyone I know who has 16GB RAM has claimed that it's more than enough. I've got 8GB, and haven't any complaints. I even know people with 32GB who say that with hindsight they'd have been perfectly happy with only 16GB.

An SSD however, is incredible. I originally wrote "essential", though I guess it's not... but it certainly feels like it is once you have one. The downside is that it will make working with non-SSD desktop machines irritating-as-hell.