We don't have for now the requierement needed for Lubuntu, because some components are still in developpement.However, I done some testing on Karmic + LXDE/Lubuntu packages updated from Lucid + some adjustements, and a session start with 50-60 Mo, according to the taskmanager. So in your case, the memory could be a bit short to be confortable.For the processor, I can't test if it's useable. But It will be a good candidate for testing Lubuntu Lucid, and I will be happy to have your feedback on it.
The choice of the browser is not final for lubuntu 10.04. It will be discuss during the cycle, and also during Ubuntu Developpers summit. You can see details on this 2 specifications :- https://wiki.ubuntu.com/Specs/LubuntuLucidApplications- https://wiki.ubuntu.com/Specs/ArmLightweightBrowser
The real first release of lubuntu should be for Lucid (10.04), in six months. To prepare it, I began to write 3 specifications  about the future Lubuntu, to hold discussion about some aspects of it. I hope to be at the Ubuntu UDS  to work on them. Feel free to contribute to them (with constructive arguments, not troll please :)) Nothing is decided, and help in various aspects to make Lubuntu 10.04 rocking, will be very appreciate :)  https://blueprints.launchpad.net/ubuntu/+spec/lubuntu-lucid-artwork https://blueprints.launchpad.net/ubuntu/+spec/lubuntu-lucid-default-applications https://blueprints.launchpad.net/ubuntu/+spec/lubuntu-lucid-packaging-policy  https://wiki.ubuntu.com/UDS