Well, considering that there hasn't been any real updates in a while, I figured I'd make a new one. I also did one of these here about a month and a half ago:
http://www.ocforums.com/showthread.php?t=424767
I might do more of these later.
Edited to include PCI cards for those who are stuck with onboard graphics and no AGP/PCIe slots.
Money is no object:
PCIe nVidia: 7800GTX 512MB
No real competition here. As of now it's the best available. At about $750, it's also the most expensive. Two of these in SLI also beat the X1800XT with Crossfire.
PCIe ATI: X1800XT 512MB
Not quite as good as the 7800GTX 512, but if you want ATI, this one is clearly the best. Add the X1800 Crossfire if you can afford it. Also, if you actually do have a budget of some kind, this is only ~$550 as of now.
AGP nVidia: 6800 Ultra 256MB
If you're stuck with AGP, this is your best bet for an nVidia card. Because of this, prices have skyrocketed to absurd levels lately.
AGP ATI: See below for X850XT-PE.
PCI nVidia: See below for FX5600.
PCI ATI: See below for 9250.
$350-$450:
PCIe nVidia: 7800GTX 256MB
An easy decision. Not as good as the X1800XT but ATI can't compete at this price range for now.
PCIe ATI: Nothing as of now, since the X1800XL falls below this price range and the X1800XT is too high.
AGP nVidia: 6800GT 256MB
These, like the Ultras, are now absurdly expensive compared to the X850XT-PE, but if you need AGP and want an nVidia card but don't have the money for a 6800U, this will work.
AGP ATI: X850XT-PE 256MB
Prices have rose on these also, not nearly as bad as the Ultras, though. Buy this card if you have AGP and can afford it. Now is the time, as cost will only continue to soar. Look at the situation with the PCI FX5600 for another example of this.
$250-$350:
PCIe NVidia: 7800GT 256MB
A great buy. Not as good as the X1800XL but slightly cheaper.
PCIe ATI: X1800XL
Prices for these have been dropping lately and will probably continue to do so. The X850XT-PE or non-PE is a little bit cheaper, if you prefer that, but not nearly as good of a value.
AGP nVidia: 6800NU 256MB
All of the AGP 6800 series are ridiculous in price, and this is no exception. Avoid these unless you really hate ATI.
AGP ATI: X850XT 256MB
Comparable to the 6800U for half the price, but this is also climbing. They can usually clock to XT-PE speeds. You can also try the X800XT if you want to spend slightly less. It's still better than the 6800NU.
PCI nVidia: FX5600
When you can actually find it, expect to pay at least $300 for this, the fastest PCI card available.
$150-$250:
PCIe nVidia: 6800GT 256MB
Falling prices for this due to the 7800 series have now made it to buy this for under $250. Go for it, or the 6800GS (really the better bargain of the two) if you prefer the lower end of this price range.
PCIe ATI: X800XL 256MB
X800XL still reigns supreme here on the ATI side but falls short of the 6800GT. X800GTO² prices have also fallen, making it possible to buy one and unlock to X850XT-PE if you want to take the risk.
AGP nVidia: 6800NU 128MB
Given the absurdly high price of the 6800 AGP cards lately, this is as good as you'll get. The 256-bit memory bus puts it ahead of the 6600GT.
AGP ATI: X850 Pro 256MB
Prices on these have fallen. This is damn near as good as the X800XL, and certainly leaps and bounds ahead of nVidia's offering here.
$100-$150:
PCIe nVidia: 6600GT 128MB
Prices on these are relatively stable, still a good deal for a budget card.
PCIe ATI: X800GTO 128MB
Grab this one. Much faster than the 6600GT in every possible way. As with other PCIe card, new generation hardware has forced retailers to cut the prices on these.
AGP nVidia: 6600GT 128MB
As with the PCIe, these are a great deal and an easy choice for this price range and interface.
AGP ATI: X700 Pro 128MB
If you insist on ATI, this beats the 9800 Pro, especially in DX9, but falls short of the 6600GT.
$50-$100:
PCIe nVidia: 6600NU 256MB
256MB models are still under $100 now and this is still acceptable. The 128MB NU or LE are considerations for the lower end of this price range.
PCIe ATI: X700 Pro 128MB
Cheaper than the AGP version, still offering sub-6600GT performance but better than the plain 6600. Choose this unless you really need the extra memory or don't like ATI.
AGP nVidia: 6600NU 128MB
Prices on these have come down quite a bit, finally allowing users with a budget in this range to buy it. It's better than the 9600XT as well.
AGP ATI: 9600XT 256MB
These are an option if you refuse to use nVidia, but won't perform as well.
PCI nVidia: FX5500 256MB
They're cheap, are DX9 capable (albeit with horrible performance). This would be my choice here.
PCI ATI: 9250 256MB
Slower than the FX5500 and only DX8.1, only choose this if you hate nVidia.
Under $50:
PCIe nVidia: 6200TC 64MB
The PCX5300 is unfortunately now too expensive. If you have enough (~512MB+) reasonably fast system memory (DDR, DDR2 or Rambus), these will do okay. They support up to 256MB. Avoid at all costs the 16MB version with its 32-bit memory bus.
PCIe ATI: X300SE 128MB
Bad performance, but having sufficient onboard memory usually makes this a better choice than the 6200TC series, unless your system memory is extremely fast.
AGP nVidia: FX5500 128MB
Just a slightly faster FX5200, but unfortunately the 6200 series is out of our budget here.
AGP ATI: 9550 128MB
Prices on the 9600NP have rose too much, but this is similar to it and better than the FX5500. Avoid the SE version with its 64-bit bus.
PCI nVidia: FX5200 128MB
Roughly equal performance to the MX440 with DX9 capability. Can't really go wrong here.
PCI ATI: 7000 64MB
Horrible performance and only DX7. This card is now about five years old and was a budget card back then. However, the 8500/9200 series is too expensive.