Mate, if you're going to go to the hassle of a kernel upgrade
why not drop in the cutting edge, the 2.5.8-pre3. Why settle for one lousy patch-minor point ? 2.4.20 => 2.4.21 ? That's just wimping out. What would the boys say ? Could you live with yourself afterwards ?
Ok I'm being a tool, but the point is that you've made an arbitrary decision to get the latest kernel. If you seek the adventure and thrill of kernel grappling and would use this miniscule upgrade as a pretext to start something, then alright, I'll shut up.
But, hypothetically, what if you found out that 2.4.21 was really the same as 2.4.20 except with some slight alterations to some device specific functionality that you're not going to use anyway, because you don't own any of the relevant devices, would you still be so keen ?
I like compelling reasons for my patching. But hell, maybe you don't
anyway the huge sticky thread at the top of this forum is a newbie kernel guide, maybe there is a good place to check.