If you've gone into bios and saved your configuration with the new memory, you shouldn't have a problem. The 4GB extension is for users with more than 4GB of ram (up to 64GB, I believe). If your system doesn't revert back to working condition when you pulled the memory and resaved the bios again, it's likely not the memory.
I think, and this is just a guess, that the kernel changes are the problem. I'm not a real big fan of rpm-based kernel installs, for a number of reasons, a big one being that the kernel developers don't give it their (even half-hearted) blessing. There are just too many assumptions a packager has to make, about your system, about the software you have installed, and about the way you want your hardware set up. That, and you have to remember to update your bootloader to reflect the changes, something rpms don't often do for you.
It's almost always better to do an install using the instructions included with the kernel, and notes on the changes mandrake made when they built the one they shipped. Kernels seem harder to set up initially, but once you've done it a couple times, you'll prefer the level of control you have doing it yourself.
As for your data, it's still there, it's probably just that the kernel doesn't match the rest of your system. Use Knoppix or another cd-based distro to start Linux, then mount that partition, and pull your data off it. Alternately, you can use Knoppix to fix your system, but that may be outside your skill level or not worth the effort if you're new to Linux. If you are, a reinstall (you're no longer a newbie when you know how to avoid reinstalls
) is probably the easiest fix.
In time, you'll see what you did wrong, and it was probably a pretty minor thing, or perhaps something easily fixed, and this kind of thing won't be that big of a deal, but it's part of the learning curve we all go through. Unfortunately, when you start messing around with kernels and the system stops booting, it's a little hard to diagnose the problem.