I've been long thinking about a DAC, exactly since my last sound card. But they're expensive. (Altho a good sound card is equally expensive!)
But since I'd like to buy a good set of cans (AKG) at some point, it'll become a necessity.
Mind you, I still think - feel, hear - that the el-cheapo on-board sound card makes a poorer sound compared to a decent dedicated sound card. But a DAC is a better investment, that's for sure.