So, I'll start
I've built a silent system and I do still notice that it runs so smoothly you can't hear if you try. Custom-built for little more than 500 bucks, best decision I could have made. I've spent hours and hours researching what goes well together so I also want to share the results and recommend some stuff.
Intel Core i5 3570 - 3.4GHz | a little ivy bridge sprinter, MUCH cheaper than i7 with just a pinch less performance. Great CPU.
MSI Z77A-G43 | I can't remember where, but I read a lot of good comments on this mainboard. Works well with all other parts. Easy configuration through Click-BIOS
Corsair 4GB DDR3 1333 | What to say, It's RAM. Enough for everything but rocket science. Don't get the cheap ones, look at the latency, looks like 9-9-9-24, the smaller the numbers the faster it is.
So much for key performance, here's more info:
BitFenix Shenobi Midi-Tower USB 3.0 | looks good, is quite cheap. If you want to save, recycle your old tower if the mainboard fits. I got this one because of the silent system
be quiet! Pure Power 530Watts | My breath is louder than this power supply
Samsung SATA SSD 128GB | Seriously guys, get a SSD. You can not believe the difference. Kachoooooom it is incredibly fast. And no sound whatsoever.
Thermaltake Macho | You will not be able to imagine the size of this thing. It's HUGE. But it fits into the tower. And you can't hear it.
To save money, I recycled my USB Hub, 2 DVD-RW drives, mouse (Logitech GS400), keyboard (Cherry, what else) and got no graphics card. Use on board graphic and sound ever since.
On lakaelo's hint I will get a GTX 550 Ti for BF3 now, I originally was looking at a GTX 760, but it's still 200 bucks. What are your opinions? Any recommendations for sound?
If you're thinking about building a pc, HOH.de is quite cheap and reliable, they even assemble the parts for only 20 bucks or so.