Life's tough in the trenches. In my last gig I fought a 2 year battle over a Windows2000 SE, SQL2000SE server with 4GB RAM. The management position was "it's worked fine for years and we are going to replace the application soon anyway". That particular server/application is still performing poorly at my FORMER
employer's facility. ;P
If you are stuck with SE for O/S and SQL then you have to turn to non-memory options.
1. Improving I/O response can help. Spreading database files across disks, segregating Log and Data Files. Faster disks, make sure tempdb is not an issue, etc.
2. Query tuning. It is a rare database that could not be improved with better indexes.
3. Are you sure, absolutely sure, positively sure, go check again --- NO OTHER applications run on the server. Not SSIS, Not SSRS, Not IIS, Not Visual Studio, Not SQL Express, ....
You will probably have to find dozens (hundreds) of little improvements. If you are lucky, management will realize the resource cost of the thing and accept the upgrade.