To build your own and get about 600Gb:
1. 3 x 300Gb SATA HDD
Link ~$300
2. RAID Enclosure ~$250
You can already see that you're above the price of the premade unit. I haven't done any research, but I'm guessing they use ATA/100 drives so you take a performance hit against the SATA if you were building your own. For what you are using it for, I don't think you'd notice a difference.
I've been looking into one of these for myself, but don't want to put the money into it. I've got 900Gb and can't address anymore heating problems into my small form factor media center.
The only advantage I see to building your own is the abilit to get a bigger enclosure. A .6TB RAID box is going to have 3 x 300Gb drives which maxes out the enclosure you are given. Building you own, you could get a 4 or 8 bay enclosure to build this thing up as time goes on and 300Gb drives get even cheaper.
By utilizing the old PC you mentioned, you could pick up a 3Ware RAID card and 3 x 300Gb drives for about $500. You could even load IIS and host your own web pages right off the thing. I've done that with my media center. I host my web pages and even installed a SQL Server for a complete development unit in a single box. Making a 700mhz machine into a personal file/web/sql/application server is not a big deal considering that the workload on each individual item isn't huge.