I know on my previous iMac (my current one was purchased in June 2010) you could assign increased memory to specific software packages
I remember that from pre-OSX days of last century, but we're talking OS9 and earlier.
Thomas A Reed's Mac Performance Guide - http://www.reedcorner.net/mpg
Instructions on using Console to analyze slow computer response [Linc Davis] -
The whirring noise is, disturbing. It could be a sign of a hard drive going. Maybe not, but I think this is a good time to ensure you have a good backup.
How much free space on your harddrive? If only a GB or two then it is far too full and it could be working hard hunting for space for your files. That also slows it down.
I agree about the slowing and the whirring possibly being the sign of a HDD going. Before you do anything BACKUP!!! Next run Apple Hardware Test in Extended Mode. If no error codes appear run AHT in Extended Mode 2-3 more times to be sure, it's not the most reliable test in finding errors so it can take 2-3 attempts to be sure. You will find AHT instructions in:
Read the instructions carefully please!