There doesn't seem to be any reason why installing an internal sound card in your system should invalidate the warranty - such upgrades are very common.
If you do decide to get an internal (or external) sound card, you will have to Disable onboard sound in the Bios.
However, today's onboard sound chipsets are normally capable of surprisingly good quality reproduction and some actually form the basis of some internal sound cards; an example is the well regarded Hercules Fortissimo 7.1.
I used the C-Media 8738 onboard sound chipset in my system for four years (excellent clean, transparent sound including 3D effects etc) with my IBM 2.1 speakers before being given a Creative SoundBlaster Live! sound card.
The difference was not all that obvious, the Creative providing a bit more "body" than the C-Media chipset. Yet the 8738 chipset (which has been followed by even better designs) could be bought housed in internal sound cards priced from around £6 upwards...:-)
The Realtek onboard sound chipsets are supposed to be even better than the C-Media examples.