The file /boot/config-2.6.18-5-486 just shows how the kernel was configured when compiling it. Changing this file doesn't influence the kernel image. I suggest to replace your kernel image package by the package linux-image-2.6-686 (with the tool aptitude or apt-get or synaptics) if you have an Intel CPU in your computer. The kernel image in this package supports up to 4 GB RAM. -- Regards, Jörg-Volker.