http://www.techspot.com/tweaks/memor.../xpmem-4.shtml
Go here and follow the instructions, but also in the General tab for your log files properties, Select memory from Performance Object dropdown, then in the list below scroll up and select Available MBytes.
The run your system for a while and load up you most intensive apps that you use often (usually games). When your done, stop the logging in the same place and look at the file you created - it will look something like this...
"(PDH-CSV 4.0) (GMT Daylight Time)(-60)","\\NONE-xxxxxxxxxx\Memory\Available MBytes","\\NONE-xxxxxxxxxx\Paging File(\??\C:\pagefile.sys)\% Usage","\\NONE-xxxxxxxxxx\Paging File(\??\C:\pagefile.sys)\% Usage Peak"
"04/20/2004 17:55:24.078","280","17.132059733072914","25.57525 634765625"
"04/20/2004 17:55:39.078","288","17.456563313802086","25.57525 634765625"
"04/20/2004 17:55:54.078","291","17.892964680989586","25.57525 634765625"
"04/20/2004 17:56:09.093","291","17.88787841796875","25.575256 34765625"
"04/20/2004 17:56:24.093","292","17.920430501302086","25.57525 634765625"
"04/20/2004 17:56:39.093","292","17.917887369791664","25.57525 634765625"
"04/20/2004 17:56:54.093","292","17.917378743489586","25.57525 634765625"
"04/20/2004 17:57:09.093","290","17.912801106770836","25.57525 634765625"
The first part of the first line of the second section you see in my file (highlighted above) is date / time, the second part is Available MBytes ("280"), the third is current pagefile usage in percentage ("17.xxx") and the forth is MAXimum pagefile usage over the last 15 seconds in percentage. This will change depending on what counters you add/use, but basically the first section describes the layout of the file. (see below)
""(PDH-CSV 4.0) (GMT Daylight Time)(-60)","\\NONE-xxxxxxxxxx\Memory\
Available MBytes","\\NONE-xxxxxxxxxx\Paging File(\??\C:\pagefile.sys)
\% Usage","\\NONE-xxxxxxxxxx\Paging File(\??\C:\pagefile.sys)
\% Usage Peak""
Try it - it helped me to discover that when playing Far Cry the reason i was getting choppy frame rates was because i was running out of RAM. It looks complicated but its not.
"04/20/2004 19:05:09.093","29","32.149759928385421","32.160441 080729171"
Yikes only 29Meg left! This is after i used this to find out that selecting High Res Textures used ALL my systems memory, and that the choppiness was because the Page file on my Hard Disk was being constantly accessed instead.