More! Couple of SBS2003 servers have spells of hitting me with daily allocated memory alerts, i.e. the warning threshold has been tripped in the monitoring. Now you can simply push that threshold figure up in the Server Management console, look under Monitoring and Reporting. Then Change Alert Notifications > Performance Counters > Allocated Memory and edit the figure up from say 2147483648 to 2447483648, i.e. another 300MB ish to go at before it starts screaming.
But if you look closely in the Task Manager and find that there are large memory allocations (170MB+) for sqlserver instances, you can adjust their figures.
Details on changing the maximum memory for a SQL instance, e.g. for the SBS Monitoring or the ISA Firewall or the Update Services can be found in the SBS team blog on troubleshooting high memory use.
There are some suggested figures to use from Susan's blog on throttling. They worked fine for me - one server regained 550MB and another around 200MB, with no apparent performance hits. Looked to me like it might be more relevant to pre-R2 servers, but that may have been coincidence.