I'm really interested in the MXL 990/991 mics, Jerome. Have you tried them. Where do you position the 991 in regards to your guitar? And the mixer (it looks different from the one you had there before) does it hook up via USB to a PC and MAC? You got me going now. Looks like a great setup, at least for beginners like me.
Mics first, then interfaces.
I have used those, and still do. They're good quality home recording mics. The guitar player in the band owns that set and we record with it quite a bit. You can set them up a couple of ways.
If you're singing and playing, and want to record both, sing into the 990 and set the 991 up somewhere around the bridge or wherever you think the guitar will resonate the most. Keep it as close as you can to the guitar, but when you're playing, make sure you don't move into or away from the mic too much. Your dynamic range will be all wonky if you do. Experiment with the 991 placement a bit to find the best effect. Use a pop filter on the 990 if you can for vocals. Doing it this way will give you some bleed, though. You'll hear the guitar on the vocal track and vice versa.
If you absolutely don't want any bleed and you want the best sounding recording, record the guitar stereo and the vocals separately. Mic the guitar with the 991 as above, and then set the 990 about 18" out from the guitar somewhere between the 12th fret and the sound hole. Pan one track to the right and the other to the left. Set the recording levels on your DAW so that both tracks are reading the same level. Hit record and play the guitar only. Sing in your head. This is a "stereo" miced guitar setup. Afterward, set the 990 up for vocals on a new track, play the guitar part back through a set of headphones, and sing. You'll then have three tracks (two guitar and one vocal) to mix down to your heart's content. You can go back and add fills and turnarounds, or whole new instruments this way if you want.
The 990 and 991 can also work as an "ambient" mics if you want to record "the room." If that's what you want, it should be set up where you would have someone sit if they were in the room with you listening to you play. I have hardwoods in my kitchen and living room, and the acoustics there are really nice, so when the wife isn't home, I set up my little two tracker Fostex box and goof around this way. It's a good way to record your noodlings, too.
The interfaces all operate in generally the same way, so I'm not pushing one brand over another. There's a lot of different features available, which is why I recommend shopping around for something that fits what you want to do. This one is mine http://tascam.com/product/us-800/, just for reference. As an FYI, with the 990/991 mics we're talking about, you are going to want an interface that can provide phantom power, as those mics require it. Most condenser's do.
The interface plugs into the computer via USB, and the computer opens up an input channel for each of the inputs provided by the interface. So for example, my Tascam provides 8 input channels, so when I open up my DAW, it sees 8 potential audio inputs, and allows me to route audio from them individually to various other parts of the software, ultimately ending up as a recorded track. This allows me to record 8 simultaneous tracks (great for drum sets or quadraphonic weirdness). If you buy a 2 track interface, you'll be able to record 2 simultaneous tracks. A 4x4 gives you four inputs, etc... Depending on how fast and how much memory (can't have enough) your computer has, there can be limits to total input. So for example, I know my Mac can handle 8 simultaneous connections, but if I took the exact same setup and plugged it into my mom's old Dell laptop, it would start to have problems after about three simultaneous tracks.
I'm not going to promote a particular interface, as your needs are unique, and you will want to find the right one for you, but I will recommend two features, regardless of what you get. The first is phantom power. This will allow you to run your condenser mics directly from the interface, and not have to provide an external pre-amp. The other is the ability to monitor directly from the interface. This is important because all digital recording suffers from latency, which is the time it takes for the sound generated by your mic to be processed and recorded on disk. If you are monitoring your audio post-process, you will notice a distinguishable time between when you strike a note and when you actually hear it back through the headphones. It's annoying and it makes layering tracks a pain. If you can monitor directly from the audio interface you won't have that problem.
Anyway, I've been hitting this stuff pretty heavy for the last six or eight months, and the best thing about it is that I still don't know enough to fill a thimble. There is an unending amount of information to learn, which to me is the best possible thing. The best way to figure all this out (and have a complete blast at the same time) is to just lock the doors one weekend and go to town with it. Set up and record in whatever kind of configuration you can think of. Compare and contrast. Write some new songs and record them three different ways. Eventually, you'll find what you like and that's ultimately what matters.
Good luck, and more importantly, have fun!
Someday we'll win this thing...