Creative X-Fi or ASUS Xonar D2.
The Siig SoundWave Low Profile PCI 5.1 Sound Card is a good one.
Yes, the Turtle Beach Santa Cruz sound card is fine for use on a computer that is used for alot of gaming.
A sound card upgrade is not neccessaey when changing your speakers.
In my opinion, the Creative Sound Blaster Xfi is the best gaming card on the market.
graphics card
good resolution , nice graphics,7.1 surround sound system
No, the sound card is attached to the motherboard of the computer.
A good gaming fraphics card will cost between $50-150.
If you do a lot of gaming consider getting an nVidia GeForce 9600 video card.
Alienware is currently the best gaming computer but it is very costly. I donno about your country but in India, a computer (specs-2gbRAM, 500gbHDD, Intel i3-2nd gen 3.1GHz procesor with 1gb nvidia graphic card) costs Rs 32000 which is really cheap and excellent for gaming!
sound card
Your computer's sound card controls the sound coming out of your speakers. When your sound card is damaged, your sound will be distorted.