Hey J.W. back to your original questions... Scarlett 6i6 or SaffirePro 14? First question back at you... do you have a firewire on your PC? If you don't know, let's assume not (unless you want to look for one). The Saffire Pro 14 is firewire whereas the Scarlett 6i6 is USB. You should have USB ports, so the 6i6 will work. If you do have firewire, then either would work fine.
With that said, the 6i6 has midi inputs and outputs. Unless you know that you will need them (for something like a midi keyboard), I might save $100 and get the scarlett 2i2 (149 at sweetwater). That's just me. It has two dual purpose inputs (mic or line for each) as well as phantom power (which you will need for the condenser). Basically, that's all I need for the type of recording I do. I typically only record one thing at a time, and having two inputs would allow you to record in stereo. I have more inputs and outputs on mine than the 2i2, but they have never been used.
Here's the 2i2 with a nice little demo video:
On whether your PC will need an upgrade.... The answer may not be yes/no. It might be that it would be better with a new pc, but the one you have will work. If you have plenty of hard drive space available, I would certainly give it a try with the computer you have. Even if you need to add a hard drive for space, that's pretty cheap. You can get a 1TB drive for about $70 or $80. That would be big enough to record a very large amount of stuff. BTW, adding a hard drive to a computer is easy to do, and it should be cheap to pay someone to do it for you.
Your SM58 is a good vocal mic. I don't know much about the AT mic that you have. It is omnidirectional which might make it more difficult to isolate the sound from the instrument, but it may work great. I don't have an omnidirectional, so I can't really say. It should be good for recording a group. If you aren't happy with the mics you have, the condensers I have can be currently bought for $80 (for a pair):
guitarcenter.com/MXL-990-991 ... 1126981.gc
Whatever software your audio interface comes with should be decent enough for getting your feet wet. I used Ableton lite for about a year before switching to Reaper. It does everything well, but you are limited to 8 tracks (stereo) and something like 6 effects. Although Ableton lite may be all you ever need, I would suggest trying Reaper and Audacity before you get too far along to see if you like them better (both can be downloaded for free). There are helpful youtube videos on most of the recording apps (the apps are also called DAWs for Digital Audio Workstation). I suggest go watching some intro videos on the various DAWs and see if something looks good to you. Some of them are really quite helpful.
Putting it all together, you could get the Scarlett 2i2 for $149, and get started using your existing mics and headphones. If it turns out you need a new PC, you could pick up a PC dedicated for that purpose and still potentially be in budget. If you just need more hard drive space, you could be up and recording for under $250 (for audio interface and a hard drive).
Sorry it took me a while to respond well to the original post. I am still moving slow (but getting better).