1. I use KDE (on SuSE).
2. You have plenty of space, so dividing by half should be fine for both.
3. In theory, that should be all you need to do. In practise, it is often recommended to split a unix installation over several partitions. Much of that has to do with disk access optimization and disaster recovery, both of which may not be quite as important today than they used to be.
4. I use vmware [webmasterworld.com] for running Windows within Linux (works the other way too, but is obviously less robust). Among other things, this allows me to use the Linux side as a testbed server, and the Windows side as a testing client, and I haven't rebooted the box for half a year now. Security under a default RedHat install is debatable, so read everything you can find about it and configure your system accordingly.