I was thinking about trying out Gentoo as well. How big of a partition would I need? I mean, I know I can resize it at any time, but I prefer going through really long processes like partitioning a 750 GB hard drive only once.
20GB is my personal absolute minimum. One of the downsides to Gentoo is that is has to keep a lot of development tools, headers, libraries, and stuff like that on your system in order to be able to compile programs. I was able to use a 10GB install on my laptop for a while but as I started to do things that required a decent amount of drive space (i.e. remastering LiveCDs, etc) I found myself running out of space. That hasn't happened since I bumped it up to 20GB.
How much
you need depends on how much you use it and what you do with it. If you just want to screw around with it a little, 40GB will be more than enough to get you a working system with plenty of room for experimentation. Just for reference, a working, up-to-date installation with Xorg, KDE 3.5, vlc, and Firefox (and other small non-essentials) installed takes between 4-5GB, depending on how you have it configured.
Also I used Ubuntu for a while. I didn't have and real problems with it other than how much useless crap it came with and updated every week and that every time it did a major update it reentered itself into GRUB (leaving me with like 9 identical Ubuntu boots, 9 identical Ubuntu rescue boots, and XP at the bottom by the time I realized how to edit the menu).
One thing to keep in mind with Gentoo is that, while it does a fine job keeping your system applications up to date, you'll have to update the kernel yourself. If you decide to go with the genkernel route, it should be as easy as running genkernel again. If you roll your own kernels, you'll have to copy the config, do a "make oldconfig," "make && make install_modules && emerge nvidia-driver" (or ati-driver, depending on your system), and add it to the grub.conf (I think installing the kernel with "make install" will do that, but I haven't tried it). Nothing too hard, takes a couple of minutes and doesn't need to be done all that often.
One nice thing about Gentoo is that they tend to be pretty conservative about updating the stable tree. Ubuntu tends to throw a bunch of the newest shit that it can find and dump it into the default installation (perfect examples: including a
beta version of Firefox 3 and being compiz-happy, which causes a plethora of compatibility issues with common applications). With Gentoo, you don't get pre-release software unless you specifically "opt-in," and if the portage (i.e. package manager) team finds a severe enough bug in a product they'll roll the package back.
Overall, it's generally small after you start going with it (things are compiled with what you want, not what you
might want or don't need), quick, and easy to update the system when you want to do it ("emerge --sync && emerge -uDN world; restart X or the system if applicable").
