How to implement closest server DNS resolution?

I am currently planning a multi-site web application with geographically dispersed servers architecture. Can anyone tell me who should I be looking for help if I want to implement something like what google is doing - when you type www.google.com, it will route you to your closest server in terms of the physical distance. I have already register a few static IP Address and I have a hosting company to map my IP to my domain name. Should I be dealing with the ISP or the hosting company for this DNS issue?