sound card query

A sound card is used to convert the digital signals(0 and 1 bits) to analog signals which is then fed to your audio equipment(headphones/speakers) through the jack usually routed through an amplifier to feed enough current. In electrical terms it is just a DAC (Digital to Analog Converter)

And I really don't recommend you buying a sound card until and unless you have a really nice pair of phones/speakers to pair up with. For aural bliss you should follow the top-down approach i.e Good Speakers/Headphones->Amplifier->DAC

What speaker/phones do you own? And what are you using currently to drive you headphones/speakers?

Generally the motherboard has a pretty decent sound card which is capable of driving any equipment that is not truly hi-fi.

Upgrading your phones then the type quality of your music files (Delete those 128kbps mp3's and invest in some good old FLAC's or atleast 256kbps) and then going down the never ending path of upgrading your source.

get Asus xonar DG for 1.7k they provide much better sound in comparison to realtek.

i am using altec 2.1

can u suggest me an external sound card

In simple word external sound cards usually provide better sound quality over the built in one of a mobo and a trained ear will certainly be able to catch difference.But u need to have good quality speaker set or hp to appreciate the difference.As for hp, good sc tends to have good built in hp amp which can be usefull to run high impedance hp.But all these crispy quality will be benefecial if u have good quality hp/speaker.There is lots of sound card in the market from cheap chinese one to T;HD.Now it depends on ur budget.