Check out if there is a 'hybrid' approach  you can take with
2003 to 2007.

The Minnesota SharePoint Users Group did a PowerPoint presentation on that.
Check it out:
http://www.sharepointmn.com/User%20Group%20Presentations/April%202007%20MNSPUG.p\
pt

Personally I would build a new windows  2008 farm. install moss  2007 64bit,
then do a database attach upgrade. You will need to reconfigure your SSP
which is not that big of a deal. I recently did this to upgrade  a 32bit
farm to 64 bit and had no issues.

I suggest a third approach.

- Arrange a separate 64-bit  machine

- Install Windows Server 2008 R2 64 bit on it (using Windows
2008 or 2008 R2 sets you up for an easier upgrade  to SharePoint 2010)

- Leave SQL alone for the time being

- Install moss  2007 64 bit

- Patch to the same level as your current farm

- Run Config Wizard and attach to your existing farm

- After you've validated that the install works remove the 32
bit machines from your farm



After you've done that for all of your SharePoint boxes we can attack
SQL

- Install SQL Server 2008 (or R2) 64 bit

- Shut down all of your SharePoint boxes

- Backup SQL databases and restore to 64 bit SQL box

- Startup SharePoint boxes and create a SQL alias for the old
SQL box that points to the new SQL box (see
stsadm.blogspot.com/.../...databases-easy-way.html for
directions

- Reboot the SharePoint boxes, or manually restart all of the
SharePoint and IIS services.

This way you have minimal downtime and you get to keep all of your
configuration, URL, etc.

One note on mixing 32/64bit in the same farm-
blogs.msdn.com/.../moss-deployment-how-to-mix-32-b\
it-and-64-bit-servers.aspx
From the link:
MOSS Deployment - How to mix 32 bit and 64 bit servers
<blogs.msdn.com/members/priyo/>priyo<http://blogs.msdn.com/members/priyo/\
>

26 Oct 2007 7:52 AM

-
0<http://blogs.msdn.com/b/priyo/archive/2007/10/26/moss-deployment-how-to-mix-32\
-bit-and-64-bit-servers.aspx#comments>

One most common deployment questions that I get is whether it is possible to
to have a mix and match of 32bit and 64bit servers in the same server farm.
In short the answer is yes, you can.

A little more explanation on this....

The inevitable scenario where you would need a 32bit server is where you
have a moss  farm and you want to crawl Lotus. Since Lotus does not support
64bit C++ API (at the time of publishing this post) it is mandatory to use a
32bit MOSS Server. You can have all WFE (Web Front - End) Server 32 bit and
all index servers 64 bit. Essentially, you must have same server level
(32bit or 64bit) in all tier of servers. You cannot have 1 WFE in 32bit and
the other one 64bit.

That guidance is no longer current. Mixed bitness in the same tier has changed
from unsupported to not recommended. Running mixed x86 and x64 servers in the
same tier in a temporary configuration during an upgrade  is fine. You'll need
to closely monitor load, since running this way could mean overloading your x86
servers depending on your load balancing scheme.

I prepare a 64-bit  Windows 2003 R2 server and trying to restore IIS backup from
Windows 2003 32-bit R2 server to Windows 2003 R2 server 64-bit

after restore IIS when i open MOSS 2007 Central administrator page... an Error
message .... service not available.