> 1) Is it usfull to upgrade a kernel (current: 2.2.12-20)?
yes. upgrade to the latest release of 2.2.X. i believe it's 2.2.19.
there have been several important security updates since 2.2.12.
you can also use 2.4.X kernels. i think you just need to update
binutils (anyhow, it's not many parts). these 2.4.X are not as
stable.
> 2) What is the most stable/effient one?
2.2.19 is what i'd go with for you. afaict 2.2.X is now in
maintainence mode and pretty much just fixes bugs and security flaws.
the culmination of the old series -- 2.0.39 -- is probably extremely
stable.