Greetings Techtre
I just finished my 805D build and I can share my results with you. First of all the Asus PW5 boards are the best option becasue of the 8 phase power system, but you can also use the Asus P5N boards or a high end Gigabit board. You are going to get what you pay for though, so make sure that the board has 8 phase power.
I know you want a case for $100, but if you spend a little more then you can future proof a bit. Get a Coolermaster CM Stacker, one of the older 810's. The cases give you lots of space and options....and they are BTX compatible.
In my 805D system I am running the Asetek external waterchill system. It comes with all you need to cool your cpu and GPU...also a chipset cooler, but you won't be using that with an Asus board. You can however purchase an extra GPU top pice to attach to the copper base for an SLI setup.
If you get the 3D Galaxy you will limit yourself right out of the gate, so I would suggest another unit. The external Asetek unit is great. I am also running full Danger Den internal setups on 2 other system, but I would not recomend that for a beginner.
Get some nice OCZ memory, I run that in all of my systems and have never had any issue when oc'ing.
Do this right and have the money for the right pieces....if not you will regret a hasty purchase.
In short...
805D = good choice (mine is stable at 3.8 with load temp around 53c)
mobo = Asus P5w or P52
OCZ = great OC memory
asetek waterchill external = great liquid cooler based on performance and what you get for the money $349
Danger Den = Great internal systems, but you have to be careful