I am currently running with 2 drives, a 90GB SSD for boot (huge difference in boot and remote access) and a single 4TB 3.5" HD for the data (roughly half full).
A friend recently gave me a 2TB 2.5" hard drive and I have a small mini ITX case with one 5.25" drive bay that I could put a 4-bay chassis into that would allow me to hold the SSD and two 2TB hard drives for data. I could either stripe the data for performance or just segment the data, doesn't really matter. I back up to a NAS, so I don't do any backup directly on the server - this protects me from the server going south and frying the drive at the same time.
The question that I have is, because the server is going to sit inside a cabinet, power and heat are a concern. Right now it pulls ~32W in idle and maybe 38W at full load.
Would 2 2.5" drives have a considerable impact on the power/cooling? If so it is worth the experiment, but if that is negligible, I would pass for now.