Computer Freezing/Crashing During Gaming?

Good chance it is the PSU. Please post the make, model and any information you can off the label on the side of the unit.

What's the chart say? should be a listing of amps/current available on the 3.3V, 5V, and 12V rail or rails.

Raymond, yes it does: And I had a detailed answer for you and some how deleted it. Here's the very very short version:
My guess is the power supply is going out. That a CWT psu has powered the pc for 12 yrs. is really remarkable, but they do eventually die.
It could also be the PSU is overheating: Have you blown it out with canned air or an air compressor (max. psi of 40-50).
I have to call it a night. I'll be back on tomorrow (being tired is probably why I deleted my first answer) and can help a bit more.

Ram doesn't usually cause crashes when it goes bad; RAM that has problems will keep the pc from booting (as a general rule). We won't rule RAM out, but let's look at one thing at a time.
Here's pretty much what I had written last night; kinda long so bear with me
CWT (Channel Well Technology) makes some really good PSU's and some that are not very good; that the CWT unit in that PC has run it for 12 years is truly remarkable!
Over time and usage power supplies wear out. The part(s) that start to go first, and usually kill off the PSU, are the capacitors. Due to current and heat they eventually become unable to regulate the power leaving the PSU. Your 420W unit probably cannot produce 250-300 Watts today once it warms/heats up.
The label show it was originally rated at 18amperes on the 12V rail; it is current, rated in amps, that is important much more then wattage. 12 yrs. ago most if not all video cards drew their power through the 5V and 3.3V rails; that's why that unit shows 40amp on the 5 V and only 18 on 12V.
Once hot it probably cannot spit-out that kinda current and it shuts down.
Does that help?
Now to testing:
The least expensive way is to borrow a KNOWN, GOOD, WORKING PSU with about the same wattage and hook it up to your PC. (You haven't told us all your specs. - we're gonna need those) IF the PC runs without shutting down, then the problem is the old power supply. If NOT, if it crashes with a good PSU, then we look someplace else, probably motherboard - which means, unfortunately, you might have to build a new faster PC.
Gotta go cook dinner; be back in about 45 minutes with a little more info.
Back.
Slightly more inconvenient and more expensive is to buy a new PSU and hook it up. Again, if the problem doesn't occur then you have solved the problem. Drawbacks: new PSU's have been known to be defective/bad out-of-the-box, so if the problem isn't solved you are back at square one. Have to return the PSU and try another new one.
Plus's: If it doesn't solve the problem and a second one doesn't solve it,then you can return the PSU and get your $$ back - assuming you bought it at Fry's or Best Buy or some other reputable big-box store.

HOWEVER, Before you go to all this trouble, perhaps you can tell the forum here what you want to do or what you do with the PC, and give us a detailed listing of the parts. This is important because of the age of your PC.

This is a long read,as I mentioned, but it should give you a good idea of why I think the PSU is the problem.

Motherboard brand and model. Usually imprinted on the mobo.
You've already provided the model of video card.
Quantity of RAM if you know it; not just how many MB's or GB's, but how many sticks in how many DIMM slots (1, 2, 4).
Only other concern would be if you had more then 2 hard drives or dvd players.
AND a budget amount and where you might shop for the PSU.
Oh Yeah, in what country do you live.

Thanks. While I dig around for a couple ideas for power supplies take a look at this list that ranks psu quality.
If I can't find anything that might be in your area (you will have to fine tune my search based on exactly where you are) then use the tier 1,2,2b, and 3 info. as a quide line.
http://www.eggxpert.com/forums/thread/323050.aspx

As I said, it might be difficult to find a good guality 300Watt unit. That Antec for $19 was the best deal I have seen for a quality unit.
Remember, power supplies only supply the amount of power that is required by the equipment that is running: a 500 watt unit is not going to output 500 watts all the time. Your system can be powered with a PSU that has a continuous maximum output of approx. 300 watts - maybe even less then that. But a power supply that must run at nearly its maximum output much of the time will run hotter and usually have a shorter life-span then one that only needs to run at 50% of its maximum most of the time.
So to answer the question: A quality 500watt unit, from Tier1-3, will work fine, but you can save a little money and still have plenty of power with 400-450 watt unit. Do a little on-line shopping to see the price differences; I found that there are deals on quality 400-500 watt units, not so many on PSU's under 400watts.