The 511 is good for at least 300 shots. Get a 1300mah at Walmart for $25, though they are on sale at the moment for $20. I always take the battery out for a recharge when I fill up a 1GB CF card with RAWs. The battery life last a little longer if you charge them while they're partialy full. Don't let them run empty. With 2 batteries, you should have enough juice for the trip. The grip only adds weight & bulk while you walk around, just swap battery while you have a late lunch and recharge them at night. Definitely take the 550EX along. Europe offers lots of great twilight and night shots. A 550EX will give you more options on exposure (2nd curtain, softer flash than the onboard flash, less red eyes, FEC for day & night fill flash, etc...) and worth the extra weight you will carry around.
Regards,
Alan