The 540s put out great sound for the price. If you're gonna use your p.c. for multimedia, do yourself a favor and buy a 5.1 or 7.1 sound card. Avoid onboard at all costs if you value decent sound, especially for music and movies.
That simply isn't as true as it used to be. The difference in noise floor and dynamic range between onboard and dedicated sound is not audible in a cheap surround setup like the X540.
(Yes, I have a dedicated sound card, but I also have much better speakers, and even so, the difference is barely audible)