You ask many questions at same time and you might break your system before long if you touch things you don't know about. In any of the versions provided, be it Kde, Xfce, Lxde, and so on : do not touch the services (that's where the tasks are located) before at least one year, and even then install a test distribution, and deactivate only one service at a time, have a session, a reboot, a new session, and wait to see what you have changed, if nothing wrong happens. All the people who did the versions took care of activating just what is needed and not more, in order to have everything working on most machines.
The Openbox Bonsai is the smallest, and was the lightest just after Lxde and Zen gnome mini, until Hootiegibbon started a new project : wmii might be even faster ! But it's not really for beginners, I think.(tiling windows... I don't know much about it yet, but you might have fun giving it a try, as live at least for a start)
What makes Openbox Bonsai a bit faster than Lxde, is that there is no desktop manager at all in it. What makes wmii faster, is the wmii program it'self, and probably some tricks Hootiegibbon added thanks to his knowledge (I think... you might want to ask him).
Browser : Hootie brought in Jumanji, that I installed and use more and more. It's so much faster than any other ! Even Midori is not as fast. You have to learn a few shortcuts.
For the learning curve, start with learning about package manager (all about installing, uninstalling and updating your applications and system, and about PCLinuxOS Control Center (all about configuring the base system), see this old document (still good) here:http://melodie.tyruiop.org/DocuPCLinuxOS/Documentation/aide-integree/premiers-pas/en/firststeps.html
newer howto's here:http://www.howtoforge.com/the-perfect-desktop-pclinuxos-2010-kde
it show Kde, but you will adapt the diverse parts...
then for lots of good readings, you will enjoy diving into the PCLinuxOS Mag collection:http://pclosmag.com
Thanks to Parnote, the Chief on board, and the team of people helping him.
PS: forgot about VLC. I found that smplayer had a nice look, and was playing audio cd's without problem. And at that moment it seemed to me that the depends were a bit less numerous. But I might change for future versions. And for just audio, I switched to DeadBeef, after Leiche had talked about it. It's really the lightest !
In the "personalized configurations" in rc.xml, you can change whatever suits better your needs, remove some, modify add... It's all there to ease the start, when a beginner tries Openbox standalone for the first time (Lxde too uses Openbox, but with a full desktop manager).