Welcome dukga.
What made you decide to try for a speaker system? If you're listening space is marginal (too small, ill-shaped, or domestically challenged) I'd recommend considering it for background use (reading, entertaining). In that case you can get satisfying in-room tunes from $100 on up. Check out the Cheap and Cheerful Circle.
If you're thinking about a desktop system, look at Audiostream.com to get a good start. You'll notice that the speakers are active (self-powered with one amp per driver). Active designs have many sonic and space saving advantages. If your headphone amp has preamp outputs you're done.
If you're after the full in-room experience start with the room (bigger is better, avoid cubes or odd shapes). Consider electrical needs, insulation, and if it will be dedicated to you (sound) or shared (with various domestic limitations). Next the likely setup must be determined (based on acoustics or domestic factors). Then shop for speakers based on room and setup. Finally (assuming you can use your source/headphone amp as a preamp) find an amp that best serves those speakers.
Floyd E. Toole's "Sound Reproduction" is a wealth of room acoustics information. Coming from headphones I'd recommend a dynamic box speaker (not planar, dipole, vertical array, open baffle, or omni-directional). The Ascend Acoustics Sierra-1 and the new Philharmonitor are both good choices for smaller rooms (neither are considered full range, that is they don't produce lots of deep bass).
Moving from headphones you'll notice reduced dynamics and "real" imaging (full in-room). Have fun.
