Welcome.
Here are a few more thoughts:
CO2 - I picked up a 10 lb tank early on, but found it does have certain drawbacks. The rig weighs about 28 pounds total, and is difficult to strap down because of size and shape. You certainly do want something that hard and heavy securely strapped when you're moving. Also, while the X may look big empty, it fills up real fast, especially if you're camping. I rarely carry mine anymore, it's just too big and bulky. I can carry a primary and backup 12v compressor in a lot less space, and the weight difference is noticeable too. I primarily use a Viair 400P portable compressor, it works well, but is a bit pricey. For about 1/4 the price you can get a copy from Harbor Freight, but there is a big difference in the build quality. If I were out alone I might not trust the HF version, but in a group where someone else could help in a pinch, it should be fine. As Bobby mentioned, they do get real hot, and putting them in the X right after using is asking to have something burned or melted unless you've got lots of free space. I just found a $10 plastic ammo can at Bass Pro that will hold one of these and allow it to cool (after I drill some holes in the box) safely. CO2 is fast, though, at least until the no-freeze regulator starts to freeze up. It's also annoying when you're close to the end of the tank, since you can't top them off. Whatever is left in the tank when you go to refill it is lost. You have to pay attention to the weight to know when it's getting low, the gauge won't tell you.
Tents - The X is pretty comfortable for 2 people. Many here use air mattresses, we use 2 closed cell foam pads each, and thick, cheap Coleman flannel sleeping bags. We've always got extra blankets around in case it gets colder. If you have a 2005 or newer there is a Rubbermaid Action Packer storage box that will allow you to extend the interior flat space to a little over 6'. It fits perfectly. Some things I have found that really make sleeping in the X comfortable are (a) Weathertech Vent Shades on the windows, so I can leave them cracked even in heavy wind, rain or snow and always have fresh air, (b) replaced the interior dome lights with red LED bulbs, and (c) a Raingler Interior Ceiling net to hold soft gear up overhead.
Fire Extinguisher - Strap it down, where ever it is. Also, if it's buried too deep in the back it might not be of any help.
Maps - I really like paper maps, it's a lot easier to see the big picture. Even the Auto Club maps have a lot of the trails we travel on them. The DeLorme Road and Recreation Atlases are great too. For places like Anza Borrego, Joshua Tree and DV there are specific maps that show all of the trails.
General Tips - like Mitch said: have fun.
