Dell Precision 390: Best Modern GPU

[Fluffster999]

Hello, Tom's Hardware Community (that makes me sound like a robot, doesn't it?),
I've an issue with my computer. As stated in the thread topic, it's a Dell Precision 390. I've invested some money in upgrading it already. It's current specs are:

CPU- Intel Core 2 Quad Q6600 @ 2.4GHz
GPU- ATI Radeon HIS HD5670 (That's the important stuff.)
OS- Windows 7 Home Premium 64-bit
RAM- 8192 MB DDR2 (It's the factory RAM, so I don't know the manufacturer)
Factory PSU, Which is 375w

Here's a link to the computer model: http://www.dell.com/downloads/global/products/precn/en/spec_precn_390_en.pdf
And a link to the card: http://www.amd.com/us/products/desktop/graphics/ati-radeon-hd-5000/ati-radeon-hd-5670-overview/Pages/ati-radeon-hd-5670-overview.aspx

I've seen a lot of other forum topics on the question I'm about to ask, but they all seem to have different answers and conditions by the asker that don't apply to me. So my question is as follows:

What is the absolute, hands-down, BEST card to put in this thing? Upgrading to a new computer is out of the question, as is replacing the MoBo. Budget isn't a problem, as long as it doesn't exceed $500 or so. I know the CPU is a bottleneck, I really should upgrade, etc... so don't tell me what I already know. I'm grateful for any help I can get with this. Thanks!

 

[Deemo13]

You wouldn't have too much of a bottleneck if you overclocked your processor a bit, maybe to 3.0GHz or 3.2GHz.

With a $500 budget, you could get yourself a new power supply, like so:
http://www.tigerdirect.com/applications/searchtools/item-details.asp?EdpNo=422166

And get yourself a nice high end graphics card, like so:
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1766293&CatId=7387

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2316880&CatId=7387

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1729168&CatId=3669

I'm just throwing some stuff out there, but with $500, the 7950 should be the best to put in it. Maybe the 580 as well.

 

[Fluffster999]

You wouldn't have too much of a bottleneck if you overclocked your processor a bit, maybe to 3.0GHz or 3.2GHz.

With a $500 budget, you could get yourself a new power supply, like so:
http://www.tigerdirect.com/applications/searchtools/item-details.asp?EdpNo=422166

And get yourself a nice high end graphics card, like so:
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1766293&CatId=7387

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2316880&CatId=7387

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=1729168&CatId=3669

I'm just throwing some stuff out there, but with $500, the 7950 should be the best to put in it. Maybe the 580 as well.

That's not a bad idea. Great, actually, but I've actually heard of problems just fitting new parts into this model. I'll take your answer as a good backup option, just kind of nervous replacing anything large with this machine (power supply specifically).

EDIT: Also, I've been reading up on this, and this model has a problem with the six-pin power connector or something like that? I think it was specifically for the GTX 460/560 series and up. Sorry for being so dreadfully idiotic, but my specialty is more in software.

 

[Fluffster999]

After some searching, I think I've solved my own problem. Upgrading the PSU would rid me of that problem. So now that I've got that out of the way, I'm thinking of balancing my budget a little more. I don't exactly want to just throw money out there, especially since it's such a high amount.

The PSU seems like a great price. So for video card, my options are pretty much limitless?
Also, I still have factory cooling on my CPU. Any ideas for what to replace it with for a 3.2 GHz overclock?

 

[Fluffster999]

I haven't been able to find a replacement heatsink for this thing. That's an area I have no experience in, any suggestions?