ATI Radeon 9800 Pro
The ATI 9800 Pro is probably the most popular and easiest ATI Windows/PC card to mod for Mac, typically the mod just works. Most of the 9800 Pros only have 128mb or VRAM, and aren't very fast. They were/are made by many manufacturers with a wide range in the quality of components. There's a large group of these cards that look the same yet vary greatly in what speeds they will handle. Speeds seem to vary from 325 to 378 MHz for GPU, and 290 to 337 MHz for memory. Finding stock Windows, speeds, and then putting days/weeks of testing into the cards to find out what speeds they will work safely and cooly at, just seems like a waste of time. The time and energy it takes to speed test/grade a card is much better spent on something like the copper heatsink version of the BFG 7800 GS OC.
I generally don't bother with 9800 Pros unless they "just happen along", and are a 256mb version of the card. The One exception to this is the ASUS 9800 Pro with a large copper heatsink and dual fans. The very nice heatsink/fan setup makes them easy to tell apart from the mass of cheaply made 9800 Pros that all look alike. ASUS built these with both the R360 GPU, and 256mb of higher speed memory like found on the 9800 XTs, they're basically 9800 XTs that have the wrong name.
ATI Radeon 9800 XT
The Wintel/PC ATI 9800 XTs are the next most popular card to mod, and in general, a much better card with more consistent quality and speed capabilities. They can be converted to mac using either 9800 Pro, or 9800 XT firmware that has been modified to fit the typically smaller rom chips found on the Wintel/PC version of the card. These cards usually have stock Windows speeds of 412MHz GPU, and 365MHz Memory. They seem to run a bit hot at these speeds in Mac, and most sellers set them to run at 400MHz GPU, and 360MHz Memory (effectively 720mhz since the memory is DDR).
Modifying 9800 XTs with Mac 9800 XT firmware typically results in fan speed controls working properly (but not always), the S-VIDEO port doesn't work, and DVI port(s) will ONLY output VGA thru use of a DVI to VGA Adapter (Dual VGA/Analog Monitor support, but NO Digital Monitor support). With Mac 9800 Pro firmware installed, the fan speed controls will NOT work properly (fan runs constantly at full speed), the S-VIDEO port will work, and the DVI port will output both Digital and Analog Signals. This gives Dual Monitor support, for 2 VGA monitors (with use of 1 DVI to VGA adapter) or 1 Digital and 1 VGA monitor.
This is a real trade off situation, NEITHER of these mods works perfectly. You can not get all the ports and the fan controls working properly at the same time, it is an either/or decision. If You need to use a Digital/DVI monitor, then you will have to put up with extra fan noise, and it's possible premature failure due to running full speed all the time. On the other hand, if you want the fan controls to work properly, you will need to be using Analog/VGA monitors, because the DVI ports will not put out Digital, only Analog with a DVI/VGA adapter.
nVidia GeForce 6800 GT
Opposed to the 9800 cards, once modified for mac the 6800 GT has fully functional DVI ports, that support both DVI/Digital and VGA/Analog thru a DVI to VGA adapter. The fan speed control works properly, so there is no tradeoff that needs to be considered.
Speeds & Pipelines
The ATI 9800 XT has 8 Pixel Pipelines, and 4 vertex/geometry engines, while the nVidia 6800 GT has 16 Pixel Pipelines and 6 vertex/geometry engines. While the Fastest relatively safe speeds for the 9800 XT are 412MHz GPU / 365MHz Memory, the 6800 GT runs the GPU at 350MHz / 500MHz with a bit of room to spare. The Memory Bandwidth for the 6800 GT is 32 GB/sec, vs 23.36 GB/sec for the 9800 XT. Shader Operations, Pixel Fill Rate, and Texture Fill Rate, for the 6800 GT are 5600 M/sec, and the 9800 XT is about 3300 M/sec. Vertex Operations for the 6800 GT are 525 M/sec, and the 412 M/sec for the 9800 XT. Using the 9800 XT as the 100% mark, puts the 6800 GT at about 137% for Memory Bandwidth, 170% on fill rates, and 127% for Vertex Operations.
Pricing ~ More is Less
As You can see, the nVidia 6800 GTs are much faster than the 9800 XTs, and once modified for Mac the DVI/VGA ports and fan speed controls all work properly. So WHY would I usually sell them for Less than the 9800 XT? It's pretty simple, the ATI cards can be modded on a Mac, most folks will not want to spend the time to do this themselves. They have to spend hours online finding and reading about the process, then take chances getting a used card which may have problems before they even get it. To make sure the card is good before the mod they would need a Wintel PC, but some folks would just chance it. For a lot of folks it's simply not worth the hours of time to do this for one video card, but there are lots of folks who will do it because the economy is bad, or they just want to see how it works. The 6800s require a DOS PC setup, and since most Mac users are not willing to go out and buy a Wintel PC, find and install DOS, figure out how to use DOS and the flashing progs, just for one video card, it simply means that demand for used 9800 XT cards is higher than demand for used 6800 GTs. Most simply put, I can buy the 6800 GTs for less, so I can sell them for less.