For your budget, it's more likely to be one server than 2. If you fall, will have to raise and not give the files to rise.
But if the rest. For distribution to 10G, only one CPU i7 2700K or equivalent (980, 3770, not a Hairdryer!).
I think will use the network card of type x520, and hence problems that slows down the network, will not.
The easiest way to squeeze 10G is to use SSD. For your money it will be something like 8*240Gb, such: hotline.ua/computer-diski-ssd/ocz_agt3-25sat3-240g/
Or vertex series, much of a difference on such a task will not.
Select a mother with 6 sata ports (sata3 doesn't matter), integrated video and a minimum 2 pci-ex x16 (actually it will be one x8, one x4).
To connect the eight screws, you need to put the controller. I recommend this: hotline.ua/computer-kontrollery-raid/adaptec_raid_1430sa/.
sil, despite the same chipset do not have to use, will hang the server. 4 ports, not 2, to still system screw fit.
Total, it turns out that in the 8x port you stick the network card in the 4x — this is the controller.
As a system file, use xfs (with default settings, if you specify noatime mount) and the OS appropriately, the new Linux (in the case of Ubuntu is 12.04). Older versions do not need to put, they may not be very good network subsystem. FreeBSD, too, do not need to put a new Linux works with a network significantly faster.
Raids do not use scatter files themselves. If the data is larger than 4 SSD, it is better to scatter randomly and separately to make a second copy for the 20% most popular files. How many copies and how many percent of the file, in reality, will need to pick up depends on load.
The memory is 4 or 8Gb, it still will fit as a cache disk it is virtually useless for this task.
Put nginx sendfile off, turn on aio.
This tips for your website, for example, mp3 or, say, online movies for file size 0.8-1.5 TB.
If you describe how much you have all files and the percentage of the volume of these files generate 50% of traffic (better yet 80% of the traffic to indicate), maybe the configuration will have to adjust.