if thinking of getting a system with this sound card: Asus Xonar DG PCI Gaming soundcard, 5.1 Channel, Dolby, Up to 105dB, Built-in Headphone AMP,GX2.5 for realistic 3D audio, Low-profile bracket included and i want to use a 7.1 channel headset. Does that mean that i would feel no difference in terms of sound if i used a 5.1 channel headset?
More than likely, it's either a USB headset, in which case you're screwed since it wouldn't use the sound card anyways, or it uses VIRTUAL 7.1 surround, which means that paying for a sound card is an absolute waste. *(Which it might be anyways - nicer motherboards with the realtek 898 audio codec are 7.1 surround with optical out anyways.)
1) Do you already own the sound card?
2) What motherboard do you have?
Like as not, that sound card is pointless - it's so cheap that the integrated sound card on your motherboard might even be better. Generally there's not much point to sound cards until you start hitting $100+ because there's little difference between onboard sound and low-end sound cards nowadays.