Ah, so that's what you're doing.

just do

uname -a

output on my box is:

Linux niglo 2.6.7 #2 SMP Wed Jul 28 04:25:36 CEST 2004 i686 GNU/Linux

but the 2.6.7 (or whatever) is the kernel you want the headers to match. You want the headers. For example (they match architectures) kernel-headers-2.4.27-2-686 if you were running 2.4.27 on a Pentium Pro/Celeron/Pentium II/Pentium III/Pentium 4.