Have you tried BootIT NG
? It's just a great utility.
Similarly, if you've got unallocated space on your disk, resizing might not be necessary. First, format the space using your favorite file system (mkfs, etc), second, mount the partition to a new mountpoint. At this point the new space would be accessible in Linux. Prior to using it however, be sure (while you are root) to set the appropriate permissions on it.
Finally, if you want that new space to be automatically mounted during bootup, you will need to add a line to the fstab file (in the /etc directory). It may sound somewhat complicated if you haven't done it before, but actually it's not too tough. Good luck with it -- J.W.