Here is the RAM limits from M$ website :
Name
Limit for 32-bit
Limit for 64-bit
Windows XP
4 GB
128 GB
Windows XP Starter Edition
512 MB
Not applicable
Windows 2000 Professional / Server
4 GB
Windows 2000 Advanced Server
8 GB
Windows 2000 Datacenter Server
32 GB
Windows Vista Ultimate
4 GB
128 GB
Windows Vista Enterprise
4 GB
128 GB
Windows Vista Business
4 GB
128 GB
Windows Vista Home Premium
4 GB
16 GB
Windows Vista Home Basic
4 GB
8 GB
Windows Vista Starter
4 GB
Not applicable
Windows Server 2008 Datacenter (full installation)
64 GB
2 TB
Windows Server 2008 Datacenter (Server Core installation)
64 GB
2 TB
Windows Server 2008 Enterprise
64 GB
2 TB
Windows Server 2008 Standard
4 GB
32 GB
Windows Server 2008 for Itanium-Based Systems
Not applicable
2 TB
Windows Web Server 2008
4 GB
32 GB
Windows Home Server
4 GB.
N/A
Windows Server 2003 Service Pack 2 (SP2), Datacenter Edition
128 GB
64 GB with 4GT
2 TB
Windows Server 2003 Service Pack 2 (SP2), Enterprise Edition**
64 GB
2 TB
Windows Storage Server 2003, Enterprise Edition
8 GB
Not applicable
Windows Storage Server 2003
4 GB
Not applicable
Windows Server 2003 with Service Pack 1 (SP1), Datacenter Edition
128 GB
16 GB with 4GT
1 TB
Windows Server 2003 with Service Pack 1 (SP1), Enterprise Edition
64 GB
16 GB with 4GT
1 TB
Windows Server 2003, Standard Edition SP2
4 GB
32 GB
Windows Server 2003, Datacenter Edition
128 GB
16 GB with 4GT
512 GB
Windows Server 2003, Enterprise Edition
32 GB
16 GB with 4GT
64 GB
Windows Server 2003, Standard Edition
4 GB
16 GB
Windows Server 2003, Web Edition
2 GB
Not applicable
Windows Small Business Server 2003
4 GB
Not applicable
Windows Compute Cluster Server 2003
Not applicable
32 GB
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The Geeks Week - Enabling PAE on 32-bit Windows Vista
The above link describes how to set up PAE (Physical Address Extension) on Vista 32 bits (It can be done in other 32 bits Windows), so that it can make use of 4GB RAM, not just ~3.5GB.
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** This is floating around the net in the form of a slim home custom OS, read about it here :
Windows Tiny2003 eXperience Edition for Slim OS and Faster XP Gaming on Windows 2003 R2 Server My Digital Life
With a 64 GB RAM limit you'll be all set for a while.