How do I determine my current kernel configuration?

The kernel configuration files are stored in /boot :

ls /boot/config-$(uname -r)

Make a copy with a different name and work on the copy.

Since you want to only make changes to your existing kernel, all you need to do is follow the instructions that come in the README.gz for "kernel-package". Which is install here:
/usr/share/doc/kernel-package/README.gz

You have to install the "kernel-package" package which is also needed to compile a custom kernel.

1. Install the kernel source package for the kernel you have installed and uncompress it. You will end up with a directory with the same name as the kernel source.
The kernel source is normally placed in this directory:
/usr/src

You will need to give yourself write access to the src directory by adding yourself to the src group done as root like this:

adduser your-username src

You will need to log out and back in for the change to take effect.

2. cd into the kernel source directory and copy the config file from /boot into the directory. You will have to rename it to ".config" which can be done like this.

cp /boot/config-2.6.17-2-686 ./.config

When you run "make config" or "make xconfig" all the settings from you installed kernel will be set and you just have to make your desired changes and compile then install your custom kernel.

Fully read through the README.gz mentioned above before you start. It list all the packages you need to install and give step by step proceedrue to compile a custom kernel. Which is what you do whenever you compile or re-compile a kernel yourself.