Hi all PC gaming gurus

I've recently decided to join the PC gaming scene after having had enough of console's inferior performance for years.
I am looking to build a dedicated gaming PC to put in my bedroom.
Setup aim:
- Run games smoothly at 1080P on a 37 inch Panasonic LCD TV
- Fairly quiet and compact, will be placed in corner of room
I did as much research as I can, and here's wot I got so far:
Graphics:
I already have a Sapphire Radeon HD 7870 OC, so i guess that'll do
Motherboard:
The smallest board possible which will support another 7870 card for Crossfire 1-2 years later
Currently considering AsRock Extreme-4, but not sure if I should go the M or normal edition
CPU:
Probably an Ivy-bridge i5 3550 or 3570K
Memory:
Not sure which is best match for my setup
Case:
This is the hard one... need a small compact one that'll fit Crossfire
Maybe something from Antec?
PSU:
If case doesn't come with PSU, then probably Antec Trupower 550W.
Sound:
Hoping for a mobo with onboard sound for my Corsair Vengeance 1300 set.
Controller:
No idea, maybe adaptor for using PS3/Xbox360 controller?
Wifi:
Not sure if I should use a PCI adaptor or USB dongle?
OS:
I assume Windows 7 is best for games?