I came across this issues on a ISA 2006 proxy server the other day. Basically, if you are logging to a MSDE database you may experience high memory usage by SQL server because the amount of physical memory that is allocated for SQL Server is not limited by default.
It is possible to limit the amount of physical memory that is allocated for SQL Server. Recommended values are:
System Memory - Setting
1 GB - 386 MB
2 GB - 512 MB
3 GB - 764 MB
4 GB - 1024 MB
For detailed information on how to set these limits, see the following Microsoft KB article