I'm in the same boat, i have 3 drives with about 5TB of stuff. So my first purchase is going to be 3 drives giving me 4TB of space, I can move stuff to free up a drive and then add a old to it and just keep moving data over.
Living Room Media Center/Server: Win7x64, ~6TB of space, SABnzbd+, SickBeard, MySQL, 32" LCD Vizio Internet TV, XBMC Nightly as of probably a month ago.
Bed Room Media Center: Mac OS X 10.6, 24" iMac Core2Duo