Read up on these topics (google) and seek help of a professional if you are in the least bit uncomfortable. You can do severe un-fixable damage to your unit if you don't know what you are doing.
I am providing info for you to provide to a tech if you need one.
Factors to consider with your specific symptoms:
1. How long have you had the unit.
2. How high/often do you overclock it.
These factors could contribute to an overall heat issue on the processors and memory which could cause damage if not properly cooled. If it's been over a year of heavy duty use, I'd consider canned-air cleaning of the entire system and regreasing the processors regardless. I assume you have a decent cooling system if you are adjusting the clock times of the RAM. If not, install or have someone install one. It's worth the investment to keep the board from burning out.
Shut off the unit, remove the 2 gig stick and turn the unit on and see if you still are having issues with everything locking up on you so often. My rule of thumb has always been to NOT mix RAM of any different types on any board. You can see a more detailed explanation here:
There are exceptions to the rule, but I prefer to keep all my RAM the same size, manufacturer, speed, etc. Just makes life easier in the long run.
C. HARD DRIVE
BACK UP ALL OF YOUR INFORMATION FIRST!!!!!! Assume you will lose everything if you don't do a backup.
To check your hard drive: If you are able to, download a linux distro like Ubuntu 11.10 and burn the iso to a stick and boot from the stick without installing it to the hard drive. Detailed instructions on how to do this can be found at www.ubuntu.com.
When you first boot into the setup screen, there are a couple utilities to check things without even loading the OS. Once you load the OS into RAM you can use the Disk Utility (System-Administration-Disk Utility) package to inspect the drive(s). I have been able to repair/ignore bad sectors, etc, on occasion using this program.
If you are getting really bad ratings on the drive, bad read/write errors, lots of bad sectors, etc, it might be a good time to get a new hard drive.
Hope this helps.