I'll confine my comments to things with which I have experience :
Mobo - I have a Gigabyte GA-MA78G-DS3H with an AMD Athlon64 X2 4850e and a Mini Ninja. This uses the AMD 780G Chipset and has on-board ATI Radeon HD3200 graphics with Gigabit LAN. Works fine and doesn't get hot with 1 fan (I have a fan at 422rpm - SpeedFan reports 45C for the CPU and 95C for the GPU). (This in a P182 case)
It advertises good audio, but I don't use it as I have an M-Audio 2496 board for audio digitising.
Antec 3480 is the smallest that will easily fit with a 120mm fan AND has 2 compartments (but if you aren't generating much heat then it's not so relevant). You're paying for a PSU with this that isn't that quiet.
I also have one of these cases with a Silverstone PSU and 1 fan running with an AMD Athlon X2 Be-2300/Asrock board. SpeedFan reports 50C for the CPU and 700rpm for the fan.
Antec 4000 is easier to work with (more space). It also has good reviews.
Maybe this is OK for your needs and you might want to spend the saved Â£ on a 3.5in drive box (see below). The 4000 also allows a full size ATX board.
A Pico 120 would be OK for your needs unless you want to add-in a better graphics card.
For general work the PC4200 is now a bit slow, I would suggest a minimum of PC5300 at 667mhz. I'm using PC6400s at 800mhz and I don't do much that is special.
If you want more than 500GB, then yes, get 3.5in Samsungs (1x 750GB or even 1TB). I suggest you put even this in an enclosure or soft mount it with knicker elastic. I have the best enclosure (GrowUpJapan 2002c - http://www.quietpc.com/gb-en-gbp/manufa ... /gup_japan
I have a Samsung in my DIY NAS in this box on a bed of soft foam - can't hear it ! I also have one in my P182 with a WD 74GB Raptor in it - also very hard to hear it, but the P182 is a bit special as cases go.