Exchange 2003 to Exchange 2010 upgrade design

Hi,
I would like your advice, for the solution to the following project:

Customer with 1 main site (HQ) and 35 remote sites. In each remote site, at this point, there is a DC/global catalog/Exchnage2003. The people distribution is 300 people at HQ and 30-50 people at the remote sites. The maximum message size allowed to transit within the organization is 20Mb and so it the maximum message size allowed to transit from/to the Internet. Also, the number of messages sent/user/day is more or less 20 messages and number of messages received/user/day is 40.

Projected growth of the user base on Exchange Server 2010, for the next 3 to 5 years will not exceed the number of 2000 users.
Outlook Web Access and Outlook Anywhere access to external users and Exchange ActiveSync access to mobile users will be provided. Also support IMAP / POP protocols will be available.

The link speed, utilization, latency and available bandwidth between central and remote sites is Adsl mpls VPN 24/1Mbps and the Central site only, provides Internet connectivity for external user access. Central site provides also the Internet connectivity for external mail flow.

There is ISA, Symantec mail security for the mail flow.In more detail, all network connection among sites and to internet MPLS VPN central connection : 10Mbps metro-ethernet
Internet connection : 20Mbps metro-ethernet

In that senario, Would it be better to retain the remote exchange servers in every site or would it be better for all the users (or some) at the sites to connect to HQ's Exchange?

Thanks in advance,
Argi