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
A good gaming fraphics card will cost between $50-150.
No, the sound card is attached to the motherboard of the computer.
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
yes it can be , depending on how much RAM you have. also you can get a proper gaming laptop with awesome sound and graphics card and heaps of RAM, but i think it cost 3000-4000 dollars