mod_rewrite

I'm building a website buider app and I'm giving users a url of the form username.mysite.com but I want to redirect all url's that end with mysite.com to a script and query the db based on username.I've learned that I need to use mod_rewrite for this.The documentation suggests that this will cause extra load on the cpu.should I be worried about this? I'm hosting with a third party.What will their reaction to this be. Has anyone done this sort of thing and is it advisable. I don't want to redirect from a 404 page as there's a search engine issue with that. As a side issue,there may be a lot of database updating involed in the site and i'm wondering should i use mysql(i am at the moment). any real world examples of the performance of mysql?I know it's great for selects. Thanx adrian murphy