Graphics Card Upgrade Advice!

Hello everyone,

I need some advice on upgrading my graphics card. I am by no means super tech savvy, but I do know the basics. There's a few problems though, so here I am to ask for advice from people who know more about this stuff than I do.

At the moment, I have an nvidia GeForce GTS 450. Not exactly top of the line but it gets the job done. The problem is that the memory has become corrupt, to the point that I have to underclock it in order for it to work with certain things. I want to eventually be able to play Diablo 3 with little or no lag once it comes out. I looked at canirunit.com and with my current graphics card and processor it passed the minimum but didn't hit the recommended specs.

So I was thinking, since my graphics card is basically done (underclocking makes certain games stop artifacting), I think it might be a good idea for me to either just put in the $110 or so to replace the card (the warranty is gone for sure), or spend a bit more for an upgrade.

My main concern is that my processor and motherboard are old. They have always been a problem. I have upgraded my RAM and power supply in my computer, along with my graphics card a couple times, but everything else has stayed the same.

It always makes me nervous when getting a new graphics card, because I'm never sure if it will work with my motherboard or processor or if my power supply is good enough to handle it, etc.

My specs are as follows:

Using Vista
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4 GB RAM
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Processor: Intel(R) Core(TM) 2 CPU 6420 @ 2.13 GHz, 2.13 GHz
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Graphics Card: NVidia GeForce GTS 450
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Motherboard make: Dell Inc.
Motherboard model: 0CT017 (Microprocessor)
Chipset Model: Intel P965/G965
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BIOS info:
Brand Dell Inc.
Version 2.5.0
Date 05/24/2007
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Power Supply: 550W
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So as you can see, my processor and motherboard are kind of crap. Are there even any upgrades to my video card that will work with them? Any advice on this would be greatly appreciated. For a price range, I was thinking something along the lines of $150-$250, but I'm really not sure what a better graphics card that would work with my desktop would cost.