What is the best plan for Exchange Server 2010 ?

Do you plan to install DC and Exchange again on the same server? That is not really good idea.
http://smtp25.blogspot.com/2009/08/exchange-2007-installed-on-domain.html

I agree with Davorin.  Having Exchange on a DC is only a good idea on an SBS where it's designed to be.  Exchange on it's own is a resource hog.  I would set up an Exchange server first and move Exchange.  Then remove Exchange from the DC.  There should be no reason to set up a new DC is you're on Server 2008 R@, but Then it's just a simple DC migration process.  if this is a small organization, have you considered  SBS 2011?  

Thanks all ( Mr. superTaco & Mr. Davorin)

i acknowledge that it is not recommended from microsoft to install Exchange server with DC on the same server.

but i joined my company and i found this enviroment there.

let me sir explain what are really there ( do not be shocked like what happen to me ;) )

there is another Windows server 2000 DC, and all the Computers joined this DC (1500 users). the new DC (2008 R2) only to give the users accounts on exchange 2010.

by default this is not acceptable at all for any System administrator. ( !!!! )
so what i am thinking is to seperate the exchange server and move it to new server.
also i need to change the server it self so i have to move also the DC to another server.

for the old DC i will do migration to 2008 R2 and shift all the users to the new Domain .

this is what i am thinking.

please advise....

ah ok, now I understand.  I wasn't blasting you, and LORD KNOWS, I have been thrown into something like that.  If you have a 2003 DC, Make that the PDC and take the 2000 server out before anything.  then do an Exchange migration to a new server, with a proper decommissioning.  then the DC migration.  it sound to me thought that the 2000 DC might be a different domain.  To do anything with a 2008 server, 2000 can't be domain functional or forest functional level

thanks for your support

by the way i do not have win 2003. and can you help me how to do proper decommission for the exchange ?
if its possible cause i need just to be sure from what i have to do, it is running enviroment .

also as you said, the 2000 DC might be a different domain

Yes.  An Exchange migration is quite simple.  

http://www.messagingtalk.org/step-by-step-guide-on-how-to-install-exchange-server-2010-beta-p

That's for the install.

this is more for Exchange 2007 to 2010, but Exchange 2007 and 2010 are so similar, this should help you move.

http://www.networkworld.com/community/node/47778

Dear Mr. Davorin

many Thanks for what you did for me.
i would to make the things clear for all, i do not need to upgrade Exchnage 2007 to Exchange 2010.

i will try what you said

again Thanks

Yes, I know that you are not doing a exchange 2007->2010 migration, but I could not find an article for exactly your case.
As exchange 2007 and 2010 are really similar, I have posted migration guide and I have noted what steps you don't need to do - as this is not migration.
If I will find more appropriate article I will post the link.