Need help for sound card..?

i got an acer mini desktop.. i plan to buy a usb sound card to improve the sounds but they said buy a internal one for better sound quality. the reason i want to buy a usb SC is bec it is very easy to install unlike the internal one which you need to fix it inside the unit and i dont know how to do it. is it that hard to do it for a newvbie like me? what things should i look at when buying an internal one? should i consider the PSU of my unit before buying one? i got a mini desktop and it is on quadcore. its actually an entry level gaming pc.. thanks!