the race header alone is probably good for between 6-10hp by itself, but you're best off getting the full exhaust system to go with it. the race header eliminates the biggest bottleneck (the stock cat) but there are still some tight spaces in the stock exhaust system that need to be cleaned up.
from what other people say, you MUST have a race header to get proper gains out of cams or even stock cam tuning. thats where extra gains will show up that you would miss without it. the 6-10 hp figure is about right, stock-to-stock, but each exhaust-related mod you do upstream of the race header (cams, headwork) will give you more HP than had you not used a race header.