Need help with my router

Hey everyone! I’ll be the first to admit, I’ve messed around with routers before, but this time I think I actually managed to screw things up properly.
A relative handed me this Linksys WRT54G that’s running that whole dd-wrt thing. First off, does anyone know how I can strip that back to factory settings and just make it a "normal" router again? Or is there even a point in bothering with that...
But then there’s the bigger headache...
Up until now, I’ve just been running on Wi-Fi—you know, antenna outside, coax cable running to a PCI WLAN card, everything working fine. Now I’m trying to integrate this router into the mix so the whole house actually gets decent coverage.
Is there anyone out there kind enough to walk me through this?
Thanks in advance.

What’s the deal with saying "everything's working fine throughout the house"—that’s a bit vague, don't you think? Have you actually called up your ISP yet?