From my experience and what I've read:
1. Which GPU is the fastest miner?
The fastest would be the HD 6990 @ ~740 Mhash/s but your going to pay $300 - $350 for it.
You can buy (almost) 3 x HD 5850's (Each over clocked to 850 Mhz core to get ~327 Mhash/s per card to total 981 Mhash/s) for that price and get a faster hash rate.
2. Can I get any CPU for the job (even a $20 one)?
For a dedicated miner any CPU will do. Usually any single / dual core CPU around 2.0 GHz will do just fine.
3. How many GB of RAM do one need?
You don't need much at all but since RAM is cheep, I usually get 2 GB
4. Should I use a small SSD as opposed to a small HDD?
What ever your preference is really. SSD is probably to pricey for a miner. My self I just use old 20 GB IDE hard drives I have lying around.
Also if your feeling adventurous you can load a RAM drive off a Flash disk with Linux running the OS and mining software from RAM, no hard drive needed. (Make sure you have enough RAM installed of course)
5. Motherboard - advice!
Stick with motherboards that have 2 - 3 x16 PCIe slots on them. So far I have 2 miners set up using MSI 870A-G54 motherboard. Haven't had a problem with them yet.
Also you never asked this but the power supply is also a very important part. Make sure you get one big enough to handle all your hardware and then some to prevent damage to your hardware or even burning out a PSU.
6. What does a mining machine cost per month in terms of electricity?
This all depends on what hardware you buy and how much your kw/h rate is for your electricity.
If you do start mining I would recommend using a power monitor to get a good idea of the usage. I use a Blue Planet Electronic Energy Meter. I got it pretty cheep, you can find better quality ones for more but this one has been working good so far.
7. Can I live with my mining PC (put in the bedroom) (i'm a student)?
It will get pretty warm quick in there. I had my miner next to me in my living room for a day and couldn't stand the heat so I had to move it to my basement where it was cooler.
8. Here's one: How many U.S DOLLARS does a mining machine (with the strongest single-card GPU) expected to "create" per month?
This all depends on the hardware you buy, the mining difficulty at the time, the current market value of a bitcoin and luck. There are a few calculators for this around but they give very rough estimates.