Personally I would go with at least a 750w , when you make a PSU work hard they tend to burn a bit more power. The card won't fit unless you remove the harddrive cages.
If you are streaming, i would stay away from speakers, you will have a lot of issues with feedback. I don't own a Blue snowball mic but it seems to be a really good choice, i will be picking up one to replace my zoom H2.
It is most likely to be a 2GB card, did you even bother to look at the sticker on the back of the card ?
Total available graphics memory is how much of physical ram you system can use when you run out of VRAM.
The only reason i have a sound card is because it has a better SNR than my on-board audio and it can provide more power to drive some higher end headphones.
Front of the card it looks pretty sweet.
Back of the card it looks pretty ugly and it is a dust magnet.
I cut and added a EK back plate, now it looks pretty sweet.
It really depends on your source, if it is a iphone it is more than enough to drive the headphones. I dont own a pair of m50s but i have a iphone/fiio e17 combo and to my ears it sounds pretty close.
my vote is for the Prodigy M also, I am not a big fan of the cooler choice, i would personally go with a swiftech option if you haven't bought it already.
if its not spinning at all it is probally the driver board and not the platters, it may be repairable if you swap the controller board but it is a gamble at best. if the data isn't important i would just bin it.
I am not sure how big the resistor is on a LNA but from pictures i am going to guess it is a 1/2 watt resistor so i wouldn't use it before the Y splitter, maybe after.
It really depends on the sound you like. I would check out Head-fi forums if you really want to learn more about headphones. I love their for sale section , some lovely deals come up often.