I'd recommend you have a look at
this thread in which some fellow members detail their experiences with various storage solutions. In your case, a WiebeTech RT5 seems like a suitable solution and those who use one have given very positive feedback. There is a new RT5e version which has just been released, but you might also be interested in
an older thread here.
To be honest, the RT5 tops out at (5-1)x1 TB = 4 TB (which are
very expensive per GB) which might barely make your requirement. If you go for 750 GB drives, you have only 3 TB of usable space.
The RT5 has various connections (eSATA, FireWire 400 and 800, USB) -- which makes it extremely fast (if you buy a SATA card with an external connector). If you don't need USB and FireWire connectivity (there are eSATA cards available for notebooks), then you should get the RT5e.
Edit: Apparently the RT5 has a 2 TB per volume limit which is apparently due to the fact that
USB cannot handle more than 2 TB at a time.