Right now I'm using a Lenovo Thinkpad T400. It's 14-inch, has comparable performance to an entry-level or mid-range desktop, and great Linux compatibility (save for the optional fingerprint reader, and the switchable ATI graphics if you choose them, since ATI drivers in Linux kinda suck, though you can always switch to the Intel graphics in the BIOS).
It has a few annoyances (EG. PC speaker beep on holding too many keys down, but you can also turn that off in the BIOS), and the Windows OS it ships with it is a bit bloated (slow startup - I'm still trying to figure out how to shed some pounds from it). On the other hand, I love its keyboard layout (much, much better than the vast majority of laptops, since it's actually similar to that of a desktop system), has good battery life (I typically get around 4-6 hours with the 9-cell battery. People claim you get get up to ten hours (!!) but I chose the CCFL-backlit 1440x900 display over the LED-backlit 1280x800 screen - the LED one is supposed to be more efficient with power), and again, great Linux support.
14-inch is a nice size. It's small enough to fit nicely into a backpack but not so small that it's forced to have weak performance, and it's large enough to sport a typeable keyboard.
Ubuntu basically == Linux, so I'm going to speak on behalf of my experience with it. Linux is a very, very flexible operating system, and it can be fitted for pretty much any purpose. There are desktop environments and window managers to fit just about any taste (from minimalist and geeks-only, to simplistic and easy-to-use design, to a maximalist "everything is integrated" experience). Linux has more useful free utilities available than any other system out there. It's definitely lacking in hardware support, but the situation is getting better. When it comes to software, packages are typically handled by the Linux distribution you are using. These packages are held on a repository for easy installation. For programs that aren't being maintained by your distro, obtaining the software is significantly more difficult, and is geared toward intermediate/advanced Linux/Unix users.
As for SRB2, I haven't really tried the Linux version. Blame Callum for not making an Arch Linux package. ;P