It's impossible to make a specific suggestion without more information.
What type of boating? Offshore, coastal, lake?
Any other electronics aboard- sounder? radar?
With a 340DA you should have enough room on you dash for either a 4000 or 5000 series Garmin. These provide from 8inch to 15 inch screens. the big diference in the is wether you want touch screen or not. Combine these with the G2 Vision card and the XM radio weather Radar unit and you have a complete system for coastal and off shore services.
On my my smaller 240DA I have the 545s do to space restrictionsand absolutely love it. the G2 Vision gave me the advanced feature in a small package that I use nearly every outing to find fuel, food, best track to a location. it's an awsome system in my opion.
check out the website and see what would fit you're budget, boat, and cruising needs.