a seperate sound controler will not improve performance in any way.
it will, however, 'possibly' make you happier when blowing stuff up!
youre onboard realtek is a 64 channel benchmark (as it is basically fitted to every mobo under the sun), everyone has it.
there are many arguements for having and not having a seperate sound controller, but it boils down to personal preference, and whether you can hear the difference.
i always say:
you ever eat baked beans? can you tell the difference between value beans and heinz? some people can, because their taste buds are more sensitive. and this is true for sound. a boom from an explosion may sound the same, but will you hear the dirt and rubble as it sprays across the floor? better sound accuracy and isolation from a separate controller/card (with 128 channels of sound / double a mobo) will allow you to hear things (if you have good hearing) better, as the sounds wont get drowned/missed out like they do on mobos.