SQL Server 2000 instance for work seemed stuck at a GB of RAM despite having 40GB of data to use & 4GB to play with (minus RAM+Supercache). Turns out my 2048 worker thread setting precluded it from passing 2GB usage limit (512K is allocated per thread). Dropped count down to 768 & told it to use 1.25GB for the database: hasn't hit that mark yet, but I'll monitor.
* Max Worker Threads on MSDN
* EggheadCafe posting I found