The true bottom line for your situation is to utilize your RME for your recording and mixing. The onboard computer soundcard on your motherboard, is 100% adequate for your playback enjoyment through your 5.1 surround sound system. If that 5.1 surround sound is important to you, playback will be adequate. It's certainly not an adequate device for recording much of anything. Still usable on the line input but certainly not the microphone input. The microphone input on the motherboard is designed strictly for a multimedia crappy microphone. You can on occasion utilize a small and inexpensive mixer to feed the line level input of the crappy onboard motherboard computer sound card. And you would only do that if he did not have enough inputs on your RME, for instance in recording a whole band at once with some multitrack capabilities. However, monitoring becomes quite a problem, during recording. That's because during recording, you will have output coming from the RME and from the computer sound card, separately. The only way to hear that is to take both of those outputs through an additional mixer.

However once you have made the recording, you could play it back in its entirety, all tracks, from a single sound device. Either through your computer motherboard soundcard or through your RME. But you cannot play it back simultaneously through both. As I said you can record through both of them simultaneously but you cannot monitor everything through both simultaneously during recording. This is a bit of a kludge factor when dealing with multiple computer audio interface devices. This is another reason why folks might purchase a PreSonus Fire Studio, FireWire, 8 simultaneous XLR or 1/4 inch input device. You could then record eight simultaneous input sources while monitoring all eight through its common single stereo output. Otherwise with multiple devices, you will need a mixer to monitor the output of those devices while recording. USB and FireWire are discrete data streams that cannot be mixed together. So you either have to deal with a workaround scenario with an additional outboard mixer? Or, you simply have to purchase a multi-track computer audio interface like the PreSonus Fire Studio or something equivalent to that, of which there are quite a few similar units. From 8 to 24 tracks of simultaneous input. But hey, if you need 24 simultaneous inputs? I'd recommend a dedicated standalone, hard disk, digital multi-track recorder such as the Alesis HD 24. And that's what I have and utilize. Both the dedicated 24 track digital hard disk recorder and a 8 analog input/24 digital input, 10+ year old, MOTU desktop, PCI-based, 2408 MK II. I also have a couple of smaller, USB, line level interfaces by Edirol/Roland & M-Audio, lightweight portable devices. Which allows me to record for simultaneous tracks, of which I can only monitor two channels from a single device without an extra outboard sub mixer.

I hope this answered your questions? Any others?
Mx. Remy Ann David