KDE vs. Gnome
w/ version 4 KDE has become a v!$t0 like unpleasantness :(
the User Interface has been completely rewritten and the focus of the developers was more on the eye candy then on the functionality which means that even after 5 (sub-)versions some basic functionalities (switch desktop, configure the desktop, menu structure aso.) are not yet readily available.
Gnome has been designed with the idea to make the switch from LostDOS to Linux / GNU a little bit easier and even though it has had its share of eye candy "improvements" too, it has remained truer to its roots.
i would thus recommend that you start w/ openSUSE's Gnome Live CD; once openSUSE is installed w/ a basic Gnome desktoop, it is easy using YaST (openSuSE's central control panel) to install KDE4 (or, @ least on openSUSE 11.3, KDE3 which is still a much more usable interface) or one of the other other GUIs available (start YaST, Software Management, under System)
a basic install from the Live CD can probably fit onto a 5 GB partition, but if you intend to install & try out other GUIs, 15 to 20 GB is advisable.
power management is not a GUI feature but a system wide attribute.
thus depending on which GUI you choose you will have a different front-end to access the power management functions; those would be kpowersave (for KDE) respectively gnome-power-manager (...)
Fedora vs. openSUSE?
where to start?
1st, nope, you can't easily switch from one to the other; you can however download the live CDs from http://fedoraproject.org/en/get-fedora-options
your best bet is to download them too & try them out, just like the openSUSE ones.
if your Laptop has @ least 2GB RAM you may want to install VirtualBox ( http://www.virtualbox.org/ ) and install the Live CDs on a virtual machine in order to be better able to make yourself a better idea of what each distro offers then only w/ the Live CDs / USBs.
personally i was using RedHat up to version 8 (about half a dozen yrs ago) but when RedHat decided to drop their "free" offerings into the Fedora community, it somehow lost its quality. SuSE has always taken much more care to integrate the open source components into openSUSE much more carefully as they want to have a good base for their professional offerings. Attachmate seems to be committed to pursue this.
changing from one distro to the other is (nearly) as radical as switching from LostDOS to Linux / GNU. you may want to make you mind up which distro you want before switching because coming back means reinstalling everything.
on the other hands, if you have enough HD space (@ least 30 to 40 GB) you can install two different distros and choose which one to start in the GRUB boot menu
(about boot menu: set your Linux install to boot from a dedicated 100 MB boot PARTITION, NOT, repeat NOT from the MBR / Master Boot Record, especially if you intend to (re-)install different distros, otherwise you very likely to end up w/ a non-booting system b/c one of the installer @#$%*&! the MBR :@
instead of creating dedicated BOOT partitions you can simply leave the /boot directory in the root ( / ) partition & set the distro to boot that ( root : / ) partition.