I would suggest a Behringer mini-mon and a used Behringer XL 4700.
http://www.fullcompass.com/product/284198.html
http://www.fullcompass.com/product/251984.html
(they are cheaper then what prices are shown if you call in)
Mini-mon will take the sound out of your sound card and do several things with it, one you can feed a headphone mix to a dedicated headphone amp, and two you will be able to control the volume of your monitors, have multiple monitors, or you can add a sub, it provides a mic to talk back to the headphone mix if your tracking room is in another part of your house, their is a headphone amp on this thing, but I don't like how it sounds that much, but would be usable if you just need to power one set of headphones etc, etc...design was stolen from Samson's C-Control, but for a third of the price, not bad....the basics of it is just a line level volume control, and a matrix switcher.
The 4700 can be had used for around $80, this provides up to four separate mixes if you have enough analog feeds from your computer, or it can just use one main stereo feed (from the mini-mon in this case) and power upto 12 pairs of headphones, I would never try twelve but the capabilities exist to do so.
Baring that,
the presonus HP-4 is a headphone amp and a volume control for your monitors, and thats it, no bells no whistles, but if that is all you need then it is probably the better buy.