You can use any PCIe sound card in the PCIe 3.0 slots, they're backwards compatible. Your onboard audio (Asus-programmed Realtek ALC1150, plus a few caps and other audio hardware) is already better than mid-end audio cards, so only a high-end audio product will actually provide a qualitative upgrade. Asus Xonar and Creative SoundBlaster products are pretty much all you should be considering, to be honest. External/USB audio devices have advantages - they can easily be moved from computer to computer, some of the externally-powered ones pack more/better audio hardware than any audio card (a PCIe card can't mount big fat tubes and amps, for example), they can be combined with other USB audio devices for upstream/downstream equalizer/amp/etc effects, and they're easier to reconfigure (with custom chips or whatnot) than a card bolted into a crowded computer. And disadvantages - their audio delivery can be interrupted/degraded by USB power/bandwidth constraints when sharing the bus with other USB devices, they invariably cost more. If you're willing to pay more (perhaps because you can use it on multiple computers, laptops, etc) and can connect the audio device alone on a USB controller/hub then external audio hardware is a good option. USB speakers/headphones seem to often cost less than their audio-jack counterparts.