Frequent Black Screens during gaming, Kernel Power errors.

I've been getting simply a ton of Event ID-41 Kernel Power crashes, during which my screen simply goes black, the sound freezes for a few seconds, and it reboots.

I've had my sneaking suspicions that it has to do with my (fairly well-aged) power supply, but this is somewhat baffling since it never did this until fairly recently during Starcraft 2, Borderlands, and now Diablo 3. League of Legends seems to be mild enough that it runs without crashing, though.

Thus far, I've installed a new OS (updated from W7 Home 32-bit, and am looking to get an 800W power supply from a friend. I know this has been popping up rather frequently on this forum, so I've viewed other threads as well.

-The mobo has no bent or leaking transistors, it's in mint condition as far as physical abuse goes
-I've kept the system dusted, nothing is interfering with power by contact
-The crashes only seem to happen during graphically intense games, possible my video card is going out to? (GeForce GT240)

Running D3 on normal settings runs fine but crashed frequently. Turning down the resolution and the gfx settings to low, it crashed much less frequently. This seems to be picking up, as I've had two crashes tonight within 10 minutes of gameplay.

So I'm wanting some input on what other viewers believe I should do here. Thank you.

The GeForce GT240 requires a minimum 400W PSU, your 420W CM PSU should be plenty from your specs. It might be time to upgrade to a Geforce GT520 2GB which you can get for about 40$ and works on a 400W PSU [ using this on a friend's PC ]

Yeah, I've had the power supply and the card both for a long time. I'll post back in this thread once I've gotten results from the PS, and I'll be ordering a new video card soon as well.

I suggested a new GPU as the GT520 is very low on power consumption [TDP 29W] compared to his GT240 and is a cheap upgrade. And my friend uses a similar CPU and a crappy 400W PSU on his 2 year old PC, just upgraded the GPU.

Thank both of you for the advice. I'm looking to actually upgrade my GPU to something more high-end regardless (looking at, say, a Radeon HD 7770), and I'll be getting the power supply from a friend free either way. So both upgrades are incoming.
Thank you, usasma, for clarifying that though.

For anyone watching this thread: My video card is potentially coming in today, tomorrow if not. I'll be getting my power supply soon after it comes in. I will have updates to this situation very shortly.

New video card and Power Supply installed (Corsair 600W). Haven't had an issue thus far, and I maxed settings on League and Diablo 3.

So.. so far so good! I'll play some SC2 (which used to crash in roughly 20 minutes of play) and, if that goes well, I'll go ahead and mark this solved. Thanks to all of you that posted!

Replacing the hardware definitely seemed to do the trick. Just ran a 3 hour custom on SC2 on high settings with no crash.

I'm betting it was the power supply over anything else, seeing as it was a stock 420W that came with the cooler master case, but in either case, the issue's resolved. Thanks again for your input everyone.