If the partition you want to merge immediately follows the good Linux partition you can use fdisk to delete the second partition and set the ending cylinder to the end of where the second partition was as mentioned in the previous post.
Once that is done you would use either the "resize2fs" or "resize_reiserfs" depending on what file system you are running. But if you have no problems with reinstalling you can certainly do it that way as well.
Of course this is all in the links that m0r gave you.
[ October 26, 2002: Message edited by: void main ]