iMac keeps locking up

[Update: No kidding right after I posted this it locked up again. doh....]

Computer Details: iMac 21.5" running OSX Mountain Lion Version 10.8.2
Processer is 2.5 GHz Intel Core i5 with 12 GB 1333 MHz DDR3 RAM

I've had my iMac for nearly a year, all has been pretty smooth until recently when it has decided to start locking / freezing up on me (more details shortly). The only changes I have made to the system so far have been #1 I upgraded to 12 GB of aftermarket RAM from the stock 4 GB and #2 I upgraded to Mountain Lion when it became available. All of this took place pretty quickly after I bought the iMac so I don't feel that any of my changes have affected as I have been running the machine like this for nearly a year.

What I did notice was there was an OS update about 6 weeks or more ago that was around the same time of my problem. Could have been a coincidence or not.

When it locks up the screen just completely freezes, nothing responds at all, and often it gets stuck on the same sound.

What I do notice though is that when I'm playing games especially more graphically intense, i.e. WoW (is the worst, will freeze up anywhere between 1 - 3 hours of gameplay) Minecraft (pretty stable for a long while, but still have had it lock up with this as well). Only once has it locked up while I was just in normal desktop non-gaming mode.

Any help or advice would be wonderful, my 1 year warranty will be running out soon as I bought it on Black Friday of last year.

bmcgrath: No I didn't notice that but I will definitely check that out next time it locks up.

Thanks for the advice, in the mean time I'll run the disk check utility and will followup with results shortly.

The DRis: Out of 500GB I'm only using 75GB. As for RAM I do run a server for Minecraft that me and my friends play on, I typically am dedicating about 1.5 - 2 GB of RAM for that, otherwise I'm just doing normal stuff, sometimes I do notice that it is using around 10GB of the 12 and I start closing things as much as I can when that happens. Last night it locked up on me and I restarted, then restarted the server which is ran via a JVM. This morning after posting this originally it locked up again.

In the meantime I'll go and run the various utilities, I've already just installed iStat here is a screen shot from that as well.

FuriousGreg: Yeah I've been debating about buying AppleCare, I think this pushes me over the top into making that decision.

Everyone: Verify Disk returns results that the Disk is OK, I'm running the Repair Disk Permissions next. Will followup soon.

Here are the results from these two tests.

Verifying volume “Macintosh HD”
Checking file systemPerforming live verification.
Checking Journaled HFS Plus volume.
Checking extents overflow file.
Checking catalog file.
Checking multi-linked files.
Checking catalog hierarchy.
Checking extended attributes file.
Checking volume bitmap.
Checking volume information.
The volume Macintosh HD appears to be OK.

Repairing permissions for “Macintosh HD”
Group differs on “Library/Java”; should be 0; group is 80.
Permissions differ on “Library/Java”; should be drwxr-xr-x ; they are drwxrwxr-x .
Repaired “Library/Java”
Group differs on “Library/Preferences/com.apple.alf.plist”; should be 80; group is 0.
Repaired “Library/Preferences/com.apple.alf.plist”
Permissions differ on “System/Library/Frameworks/CoreGraphics.framework/CoreGraphics”; should be lrwxrwxrwx ; they are lrwxr-xr-x .
Repaired “System/Library/Frameworks/CoreGraphics.framework/CoreGraphics”
Permissions differ on “System/Library/Frameworks/CoreGraphics.framework/Resources”; should be lrwxrwxrwx ; they are lrwxr-xr-x .
Repaired “System/Library/Frameworks/CoreGraphics.framework/Resources”
Permissions differ on “System/Library/Frameworks/CoreGraphics.framework/Versions/Current”; should be lrwxrwxrwx ; they are lrwxr-xr-x .
Repaired “System/Library/Frameworks/CoreGraphics.framework/Versions/Current”
Group differs on “private/var/db/GPURestartReporter”; should be 0; group is 80.
Permissions differ on “private/var/db/GPURestartReporter”; should be drwxr-xr-x ; they are drwxrwxr-x .
Repaired “private/var/db/GPURestartReporter”

Permissions repair complete

I've done some testing and it seems this definitely ONLY happens when I'm doing something graphically intensive and / or I have a server running and taking up lots of memory. It seems to me it is either a memory issue or a graphics card issue. Though sometimes it locks up without having a game running, but ONLY locks up if I'm not on a game when I have a server instance running which is using a lot of memory.

What do you guys think?