Lag in games for first time in 18 months

After a year and a half of my laptop delivering stellar performance, I have suddenly started to experience heavy lag on games (Minecraft, Half-Life 2, Portal, Amnesia). I have always been able to play them on the highest graphics settings with no problems, and now there's lots of lag and sometimes the sound cuts out for a while.

I sometimes play graphics-intensive games on the Windows Boot Camp partition, and there I experience no lag. So it looks like the GPU itself is fine.

Any idea of what I can do?

Some images of the Activity Monitor:
Activity Monitor - Imgur

MacBook Pro (2009)
OS X 10.6.6
3.06 GHz Intel Core 2 Duo
4 GB RAM
NVIDIA GeForce 9600M GT (512 MB)

I would say your primary issue is not enough free space on the drive and what free space you do have is very fragmented.

You're down under 20% free space (some long time Mac users will contest my experience). But, I'm going to say if you're playing intense games that will also use a large amount of virtual memory, at 20% free space and especially on a Mac where you've never done a defrag, your virtual memory is scattered all over that drive.

I start noticing these issues myself at about 30% free space and in my past hardcore gaming days it became unbearable by 20% free space - and I contend it carries over into OS X the same as it does in Windows. Today, I attempt to maintain 40% free space on system drives of all sizes up to and including 1TB in size. I have never put an OS on anything larger than 1TB, so don't have any experience related to keeping a system running full bore vs free space on those 1.5-2TB drives.

You would probably see some improvement with a defrag, but with what you're doing - I would encourage you to free up some space. Try to get to 30-35% minimum and either run a defrag or create a bootable backup and restore your internal from the backup - this will in essence do a defrag for you.

Along with the above, if you haven't been doing any sort of maintenance, it's time to run a Repair DisK, Repair permissions and grab Onyx for some housecleaning - all these before the defrag or restoring from backup.

edit: If you have any questions about any of these steps, feel free to post back.