As the title says, i'm trying to figure out how much RAM is needed to generate and export to excel a large report using SQL Server Reporting Services on Windows Server 2003.
It is not an option to upgrade it to SS2008 and also not an option to export to CSV.
Strictly from a hardware point of view what is a good configuration for a high load server? (CPU's, RAM, Storage)
You've got problems - the maximum memory size that SSRS2005 can handle is 2GB. (There is a dodge to enable it to handle 3GB, but it's not recommended for production servers.)
SSRS2008 has no such limitation, which is why the normal response in this situation is to recommend an upgrade to 2008.
If your large report won't run on a machine with 2GB available, it doesn't matter how much RAM (or other resources) you put on your server - the report still won't run.
Your only option (given the restrictions stated above) would be to break the report up into smaller pieces and run them one at a time.