Question about plugging headphones into soundcard/amp, need new speakers

Im using an old pair of Boston Acoustics BA745 2.1 speakers with my rig that i built, connected to the integrated soundcard that comes with my gigabyte ud3r motherboard. The speakers are connected to the soundcard through the back of the computer, and the right satellite has a headphone jack. Now when I plug in my headphones into the headphone jack on my speakers there is a clear difference in sound compared to connecting straight into my soundcard on the back, there is more bass and and a warmer less shrill sound, and it goes WAY louder. This leads me to believe there is a small built in amp inside the right satellite, i think.

I want to upgrade to a new set of speakers, probably going for a cheap T-amp/bookshelf combo once I make some space, thing is I wont have anywhere to plug in my headphones. The amp for the speakers would go into my 3.5mm on the back of my computer, (there is only 1) and the amps ive seen that do headphones and speakers are pretty expensive, or more than im willing to spend at the moment. I could just unplug the amp from the back and plug in my headphones, but plugging in directly to my soundcard seems to sound very bad, and it would be pretty inconvenient to have to plug into the back.

What options do I have for powering new speakers and getting better sound from my headphones? If i buy a soundcard that say has more than 1 3.5mm jack, can I have the headphones and speaker amp plugged in at the same time, and sound would go through to both the speakers and the headphones? Then I would just have to switch off the power to the speakers if I want to use my headphones? or would that not work, or should I just get 2 amps, one to power the bookshelfs and one for my headphones? Im using some sennheiser 428s but Im going to upgrade to something in the 100-150 range in the near future so I want something priced for headphones in that range.

I hope you guys understand what im asking for, because im not really sure myself lol, im probably making it more complicated than it seems.

Thanks for the response

ive been reading around and looking at usb dacs, so you can plug your speakers into them as well? For example i was looking at the nuforce udac2

http://www.nuforce.com/hp/products/iconudac2/

and it has rca jacks at the back and a headphone jack in the front. If i get this amp for my bookshelfs

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=310-300&FTR

would i just do a rca to rca connection and it should work? So it would be like Computer>DAC>T-amp>bookshelfs, and i can still plug my headphones in through the front? Would the volume knob control both headphones and speakers?

Also the amp you posted looks nice, I cant seem to find a retailer for it. Also it only has line out and coaxial, im guessing I can get an adapter for coaxial and rca?

Last thing (sorry) about the bookshelf speakers, What type of space/placement is recommended for them? Id be placing them on my desk but Ive heard having them close to my monitor could cause a problem, and if they have walls or other things too close to them it would affect the sound.

Thanks in advance