The best way to hook up my iphone 3G to Marantz SR8002 Receiver?

This isn't exactly what you are asking about, but you might prefer this. The iPhone can control iTunes on your home computer wirelessly. So, if your computer is now hooked up to your receiver, all you need is to download the free iPhone app Remote. Then, your phone displays all the music in your computer's itunes and lets you play them out your computer, to your receiver. That usually gives you a larger library of music than would fit in your iPhone, plus your phone is not shackled to your receiver - its in your hand in your easy chair. You can control the playback volume and everything.

Since your Marantz SR8002 does not have a USB input you will probably have to go with using a PC or a docking station to send a digital signal to your receiver.

This is how I have connected my 4G iPod Nano (same connection as in iphone 3G) without any complex network, media server setup and the music can be controlled by Harmony or the remote which came with the dock. Total cost.. approx $15. Get this and connect the audio/video via traditional RCA audio/video jack to your receiver. The dock is capable of charging iPod or iPhone via the USB. Note than many older dock system are not able to charge iPhone or iPod Nano 4G (because they charge via the firewire port, which are not present in iPhone or iPod Nano 4G).
I bought something like this:
http://cgi.ebay.com/AV-Adapter-Dock-...d=p3286.c0.m14

Oh yes, you will be able watch videos too.