I have 2 500gb drives
I now, of course, realise this is a slightly silly configuration but I have to live with it. OS, applications recorded TV, etc on first; "My Documents" and any other data type stuff on second (as far as Windows will allow at any rate). Not great, but it will do until I get a new PC.
Backups taken via Cygwin/rsync driven by a batch file, pretty sweet really. I probably should look at a way of compressing the backup, but it's not too much of an issue just now.
Initial backup took, oh, 2 hours or so. Subequent backups are much faster (sub 15 mins at times). All depends what's changed.
I think the next PC will have a 32 or 64GB SSD as primary and another HDD for actual data. Of course, it won't be running the vomit that is Win7 (which is doing a good job of pissing me off royally at the moment).