Exchange 2003 Ent upgrade to XCH2010

You can upgrade to Exchange 2010 Standard Edition. The databases are not mapped to individual mailboxes. Exchange mailboxes are stored within Exchange databases. These databases have storage limitations between the two versions.

You can find here some of the differences: http://www.microsoft.com/exchange/en-us/licensing-exchange-server-email.aspx

When instaling Xch2010 I do not join the Present organization, do I? In order word, the xch2003 won't a warning stating that I can not add a Xch2010 or someting like that?
I need to know this before purchasing xhc2010 std or ent.

When you install Exchange 2010 it will create a connector so the two can communicate. Only after you transfer all your mailboxes, PF and you are sure everything is working properly will decommission the old exchange. Here is a Step by Step Migration:
http://exchange2k10.wordpress.com/2010/01/17/transitioning-from-exchange-server-2003-to-exchange-server-2010-1/
http://exchange2k10.wordpress.com/2010/01/17/transitioning-from-exchange-server-2003-to-exchange-server-2010-2/ 
http://exchange2k10.wordpress.com/2010/01/18/transitioning-from-exchange-server-2003-to-exchange-server-2010-3/ 

Thank you very much for those links. I will get back to you once I implement the new server.

No problem. Good luck on your migration.

I am waiting to implement the server, I can't accept the answer yet.

No problem. Take your time.

I finally installed Exchange 2010 SP1 and all went really well. Most of the instalations parts one has to do manually wiht Excahnge 2010 installaion are automated now with Exchange 2010 SP1.
Your links were great thank you.