Changing Servers and Effect of Google Spider

[flashescape]

Ok I have searched but keep finding different answers to the question since it seems many people switch between static IPs so 301 redirect is not possible with me.

Here is my situation:
I have Server A with 2 name based sites and I am being moved to a new server... Server B where I have created essentially its duplicate. The two servers have different IP addresses. So I have control over the DNS A Record to basically point the two sites to the new server and wait for that to propagate.

My question is how long do I leave both sites in sync for google to properly spider the new site so I don't lose my rankings. The content won't be changing nor will any directories. They are essentially duplicate but on a new server. I have read that google's spider has its own DNS cache and I am at the mercy of that.

I don't want to be penalized for duplicate content either if that is at all possible. I was planning to submit the record change at the DNS on a friday night. Leave both servers up running for 72 hours then bring down the old server. If googlebot is still going to the old site though and finds there is nothing there due to the old IP address will it lookup the new DNS information or do I have to wait until it finds it before I bring it down?

Any help, suggestions or hell even links to articles with experience in this situation would help.

I hope I just made sense... thanks in advance to you all.