CPU - For this kind of build you are going to want a relatively decent processor. Most quad cores would be fine, an i5 would be great.
RAM - Plenty of RAM is good too, especially if you want to run a lot of VST's and/or do a lot of layering. I would aim for 8GB but if you can afford 16GB, go for it.
MOBO - Basically anything that fits your parts should be good here. It is nice to go for something with a Firewire port though because a lot of audio interfaces use firewire. USB 3 and SATA 3 are good to have too. You won't need to worry about onboard sound because you should be running an audio interface.
GPU - Basically any modern system is capable of adding a GPU so long as the PSU can handle it. I would try to avoid going for something too powerful though, to keep the noise down. You don't need one at all for audio anyway. It is nice if you want 2 monitors but Intel HD can handle that.
PSU - I would try to keep the whole system under 300-400W or so if possible. Low power consumption = low heat, low heat = low noise. This is definitely the type of system where you are going to want ultra low noise since you will be concentrating on listening to recordings etc. so having a jet engine sat next to you is going to be very annoying. Also you wont want fan noise in your recordings when you record with mics.
CASE - I would go for closed style case with less vents and spaces for fans etc. These are places where the noise can escape from the case. So long as you keep the power consumption low, you won't have to worry too much about cooling. You shouldn't need a massive amount of space so a mATX
HS/F - I would definitely go with an aftermarket CPU cooler if the budget allows. A better heatsink means the fans don't have to work as hard to cool the system so you maintain low noise.
If you could fill in this form I can give you some more specific suggestions.