I've used G-parted before which i would recommend http://gparted.sourceforge.net/
. Last time I owned Partition Magic was around 2002.
Regarding size: Offical Vista requirement website says 40GB harddrive as requirement, but only 15GB of empty space (how do you install an OS on anything but empty space?). 40GB seems to me (unless you install a ton of applications), just fine. I don't think Vista or XP handle a different "Program Files" partition well with some default application installs, but you might know more about that than me. If you aren't running into size problems with the 450GB, it probably won't matter which way you do it. Unless you have a OEM (1 time install) Vista, it probably would be better to go on the safe side and make Vista partition bigger, since that would be safer to resize and fix if something went wrong when there's more data on the disk.
Partition Magic is about $70, and that money is better spent backing up your data with an external HD, which you can get for around $70 if you don't already back up your data.
As long as you don't resize over anything but empty (unused and unfragmented) space, nothing should go wrong.
My disclaimer: I don't really know what I'm talking about, but I like to try and help.