It may be easier to re-assign your ex-Windows partitions to something else, rather than combining them with existing ones. That way, you won't need to backup anything, and you'll still have the advantage of multiple partitions if something bad happens to any one of them.
Having /, /home, and /usr is pretty minimal. You could keep a small one aside for /boot, maybe one for /var, another small-to-medium one for /tmp. I've set aside a large partition on my disk for /pub, which I use to share stuff between users. Then of course you could have a swap partition if you want it, and maybe a separate /var/www if you're doing web hosting.
After you set all of them up to be mounted automatically, it'll look like one nice big drive.
Just my $0.02
