Overclocking is not great and causes allot of kernel panics under high load. Also the biggest bottleneck will be the USB controller hammering drives and serving data over LAN.
You could offload the Software raid onto a nice USD Raid controller. I am not sure exact models of your external drives but if they have eSATA you can use a cheap converter cable to connect them to this. This will give you maximum Hardware RAID performance, offload the CPU and only USB bandwidth for data and LAN, maximising read speed.
One problem with RAID0 is that it doesn't really double speeds on soft or cheap hard raids, its actually pretty rubbish. This is why ZFS was made and its supported on Linux ARM. The only problem is RAM and it may run out kernel address space if hammered, but with tweaking you will get MUCH better read and write perfomance on USB attached drives. This is because ZFS can read and write to each drive seperatly using RAM cache (this is why RAM is essential) so parts of files stripped across drives are read at full speed from each drive, assembled and served at a low level. So if you really want performance you should use a micro server/ PC with 4GB (3TBX3TB) but more is better. The results for software spanned drive pooling will blast you away. I use 4X1TB @ 4GB with ZFS on my HP Microserver and its mint! Copying from the server saturates my 100mb LAN and these are cheap, mixed set drives