Sonar 4 is an excellent choice.
I do most all my stuff myself, so I got the M-Audio 2496 a couple years ago. Since I don't record more than one track at a time, it works just beautifully. You can get the newer one, with 192, but you don't "need" it. Realistically, I actually only record at 24-bit, x 44.1. I don't even use the 96K. I tried it, couldn't hear a difference, so I opted for the smaller file-sizes. Definitely record at 24-bit though.
I have the Digitech GNX3 for my guitar. Same thing basically(options-wise) as your Boss. You can run the Spdif straight in off that for recording. I have a hardware Behringer mixer that I just run the stereo cords(L&R) from the back of the GNX to the mixer. I know you go D to A to D, but I like to be able to control everything, input level-wise, on the hardware mixer. Otherwise you will have to control your SPDIF on the M-Audio mixer inside the comp. No big deal really, just not as fast as I like.
I haven't really looked into the Boss, but the MIDI on the GNX is ONLY for adjusting patches via your computer and importing patches from the internet to your Pedal. I'm pretty sure that is standard.
If you are planning to sing at all on your tracks, you will need a pre-amp. The easiest way to do this is to buy an outboard mixer, as I did. I bought the Behringer 1604, as I have a bunch of shit I leave plugged into it full time. Then I can just move the sliders when it's time to play with that instrument. A lot of folks here will knock Behringer products, and suggest a Yamaha mixer or something. Do a search for "best mixer under $xxx". I'm sure you'll find plenty of opinions. The hardware mixers don't have the "best" pre-amps in the world, but they will get you by for a while.
Since you already have a keyboard, and if indeed it has MIDI on it(I assume so) then you are already good to go. Even if it doesn't have the best sounds in the world, you can always get software synths, and then trigger those sounds with your keyboard.
I've said enough I guess. But when buying gear ALWAYS look ahead! Do you ever plan on tracking two of you playing and singing at the same time? Or maybe tracking a drumkit? As your setup would stand now, with the Boss and a 2496, you could record the guit on SPDIF while singing on the Analog ins to the soundcard. You are good there. But, what if a friend wanted to play along also? If you don't mind tracking, basically, one at a time, it's a great soundcard. I'm the only one in my studio, and I've found it's really hard to play guitar and keyboard at the same time.

So kind of keep in mind what you are planning on doing, in the FUTURE. If micing a drumkit is in your future, you'll need a different soundcard. If not, the 2496 should suit you just fine.
So, in your situation, I would suggest getting the 2496. I would also suggest getting an outboard mixer that is within your budget so you can use the pre-amps for your mics for vocals. For a first Mic, I would suggest the Shure SM57 or SM58. The SM57 has been around forever and is a good starting dynamic mic. If you are going to get a condenser mic anytime soon, make sure your mixer has phantom power.
Just FYI, the next "step-up" in pre-amps, at least suggested around here, is an FMR RNP for around $500. So your mixer pre's will get your through your learning just fine.
Now you have a studio. Enjoy it!
