As you have a 5gen ipod the warrenty gone, so i suggest doing a DIY job cos I don't believe its worth a lot to put in a new hard drive. Saying that, it depends how confident you are to open your ipod, put a new HDD [ hard disk drive ] in, then pluggin the iPod into the comp then restoring it...
Plus if you get the 6oGB HDD, you'll need a new backing case cos they're obviously larger !!