probls upgrading ram on a 2.4 linuxbox

Hi all!
As subject I upgraded my box from 1GB ram to 2GB ram; bios sees all new memory but kernel doesn'load (error in memory stack). I have the kernel set with CONFIG_HIGHMEM4G.
I googled everywhere but didn't find any similar problem. Anyone has a suggestion?
linux 2.4.18 (slackware) on 2x1.6 athlon processor, ram266Mhz no ECC.

Regards,
Marco Marabelli