A headphone amp is only going to be useful if your sound card severely under-powers your headphones (ie headphones with 64ohms+ and sound card that only outputs up to 32ohms). The main reason people get headphone amps is for phones and mp3 players to save on battery power but using them on PC's with integrated sound (ie realtek chips output only 32ohms) can help out low sound volume.
Buying a DAC is pretty pointless unless you're going to get one that has tube amps, and on it's own will cost as much as a mid-range pc ($600+) if you can find a deal on one. Digital DAC's will only go up to 24bit-192 and I bet your sound card can already do that (as can most realtek chips).
I get the feeling you want more bass? I can't really think of any other reason to upgrade than being able to feel bass rather than hear it, and like I said you should get a home theater system with a dedicated subwoofer. The cheap way would be to get whatever logitech system that is under $100, but if you want to spend some serious money then go to musiciansfriend.com and find:
-Unpowerd Reference Monitors
-Amp + crossover
Read up on ohms, amps, and other things on wiki before you buy anything else, these are basic concepts that anyone should know before buying high-end audio equipment so you don't get ripped off or blow up your expensive purchase.
Personally I built my own system with two 15" speakers (50hz-15khz) and two 15" subs (20hz-400hz), grabbed a decent amp+crossover and have used it ever since. It's not cheap ($1,000+) but I needed it for bass guitar so I could be heard live.