Help with install, Infiniti and double din
 

car: 98 Infiniti Q45
stock stereo: Bose stereo
What I want to put in : JVC KWAVX640 double din

What will I need?

Do I just need a regular Nissan/Infiniti wiring harness adapter?

If so, is this the correct one?
http://img832.imageshack.us/img832/488/adapterg.jpg

Description of the adapter:
Product Features:

Plugs into car harness at radio
Power/4 Speaker
One 10-pin power plug and one 6-pin speaker plug
Allows for the installation of an aftermarket radio using the existing factory wiring and connectors
For select 1995-2007 Nissan vehicles

Connections:

Yellow – 12V Constant
Red – 12V Accessory
Black - Ground
Orange – Dash Light Dimmer/Illumination
Green – Left Rear Speaker (+)
Green with Black Stripe Left Rear Speaker (-)
White – Left Front Speaker (+)
White with Black Stripe – Left Front Speaker (-)
Purple – Right Rear Speaker (+)
Purple with Black Stripe – Right Rear Speaker (-)
Gray – Right Front Speaker (+)
Gray with Black Stripe – Right Front Speaker (-)

Do I need anything else for the install like a double din dash kit?

I think you need an amplified harness (With rca's) if you have the Bose system. The rca's should plug into the factory amplifier. You may also need an antenna adapter. (for am/fm)

I'm sure soundsgood will have all the proper parts and advice you'll need.

PB is right on all accounts. You can run speaker level into the factory amp but it will be noisy and not sound that great comparatively.

Nissans of that era can accept a high level input to the Bose system without issue.