My current GPU is the Gigabyte 660ti OC, and to be perfectly honest, it's a GPU that's faster than the first PC I ever had (P3 Celeron). I needed support for the Mercury Playback Engine first and foremost (my computer is not used for gaming at all). GPU speed only has to be capable of rendering and rendering fast. I could have gone for a beefier card and it would be faster, but the fact of the matter is that the 660TI is such a huge step over my previous build, that I'm satisfied by not emptying my bank account to have something just a little bit faster, where it's entirely possible I wont notice a difference in performance at all.
The only reason I upped it slightly was for gaming at >1080p and/or if I was running across multiple monitors. 660 Ti would be enough for my work side for sure, but maybe not for play as well.
The Source 210 is not a case for dampening at all, as kuzzia mentioned. That said, there's nothing stopping it from being a silent case, it simply involves picking the right components to go inside. With the system fans soft mounted, and PWM controlled, the fans are not loud, and taking out the stock fans made the single biggest difference in noise. Cutting out the grill would be equally important, especially with tight grill spacing in the 210.
I also believe my ambient noise levels would make pushing for an absolute silent PC obsolete, but if I aim for as quiet as possible I will be more then happy.
I would honestly forget about dual CPU fans for your use. You likely will not see any difference in temps, and you will hear a difference only having one. I would use a Slip Stream 1200RPM PWM version fan for the rear exhaust. In addition to that, you can use the NoiseBlocker M12-S1 for the CPU fan (note: singular). The PSU and GPU will have their own cooling fans, so there's no reason to change that unless you're going to change the GPU cooler entirely. Depending on the cooler, I would add an 800RPM Scythe Slip Stream for the cooling on that. I would leave it at that. There's no reason to stick 4 or 5 fans into a system with so little heat output. My Dual Xeon workstation will have 4 system fans (2x CPU, 1 rear, 1 top) and the dual fan GPU cooler (stock cooler) and the PSU fan. There's no reason to stuff extra fans in there, because the existing ones do the job quite well. Given you're spec'ing out a machine with less heat output, I really have to emphasize that I don't think you're going to need more fans than are in my computer, especially if you're looking for silent operation.
Ok so I am looking at single fan coolers to fit a NoiseBlocker M12-S1 and a SlipStream 1200rpm PWM as the exhaust. I agree about the number of fans, ideally I would like 1 exhaust at the rear, 1 on the CPU and 1 or 2 at the front all going in the same direction towards the rear of the case. And I need to seal up the top of the case, I use to get a lot of dust and noise in my P182 and I don't want that to happen in the new PC. I will try out the stock fans first, but I'll pick up a NoiseBlocker for the CPU.
I'm also going to second CA_Steve with his mention of the MSI Twin Frozr models, they are very exceptional cards, and I have used several in customer builds, and they are all very happy with them.
Ok I am sold on them, MSI GeForce GTX 670 PE/OC 2GB Graphics Card. Now just to find them for a good price!
If you're building an ATX system, I would look at some Antec cases. For example, I have some Antec 300 cases lying around at work. I remove the front fans, swap out the rear fan for something soft mounted and quieter (slip stream usually, but something quiet that's available) and I look to seal off the top vents. Minimize the necessary components that need fans spinning, or use large tower heatsinks with slow RPM fans and even a simple case like that will be significantly quieter than it was out of the box. Any case can be silent, it's mostly about how far you're willing to go to make it silent.
I do have a soft spot for Antec, I think every case I've had over the past few (10+) years have been Antec. I don't mind doing a bit of tweaking here and there, but I am not overly into case modding where you can't recognise the case after you are done with it or it takes you so much time and effort where you could of spent a little extra and you get a much more capable case out of the box. My thinking is if I get quiet components and stick them in a quiet case it's like the best of both worlds, instead of sticking loud components in a quiet case or quiet components in a standard/loud case.
Also, have you thought about going mATX? The Silverstone PS07 is probably the best mATX case I have ever used, and I can't say enough good things about it. I know you're looking to have 6 drives internally, but what about reducing that number? If you really max it out, I see room for 5 total hard drives in a PS07 without modifying anything. Introduce some creative engineering, and you could change that. Even with the PS07, I would not use any front intake fans, I would just use a good exhaust fan (800-1200RPM) and it would be able to cycle plenty of air through the case, keeping everything very cool (yes, even 6 drives + SSD).
I did actually do a funny dance between ATX and mATX, so you can imagine how many times my spec kept changing. I just had to make a decision and go for it, then after a few more revisions I had to post on here because I was beginning to go a bit crazy. I think next time around I will go for mATX especially after seeing how thin and compact they are getting, but this time round I better stick to an ATX build or I will go around in circles for another month or so, bigger case with loads of room to move around and when I do replace it I can make it into a server for storage.
Building a silent PC will require you to make trade offs over what you really want and what will work in a silent configuration.
I can easily live without silence, like I mentioned there are so many other things around that make noise. Late at night however it would be nice if I could work or play and not have to listen to all the fans in my case as they try to cool the hard drives or when I am gaming.