Extending a system partition

I need some help… I am working on a clients machine that has a AMI Enterprise 1200 raid card, with two internal 18 gig disks and a external array with 4 disks. I need to do a fresh install but it will not allow me to install to the complete 18 gigdisk. So I created a partition and now I would like to extend it to use the full amount. Or tell me how to get it to use the whole disk on install.