Radeon X1950 PRO Crossfire / Geforce 7900 GS SLI
It is with some hesitancy that I recommend dual-card setups in this price segment. Dual-card setups introduce a lot of unpleasant factors that the user has to think about: they run two times hotter, consume twice as much power, offer inconsistent performance compared to a single card and may not work with dual displays. More expensive motherboards and power supplies have to be purchased. The list goes on.
But if the user knows what they're getting, it might be right for them. Two X1950 PROs or 7900 GS' can perform as well as a $400 X1950 XTX in many cases, sometimes even better. The X1950 PRO is the faster card when competing against the 7900 GS one-on-one, but SLI seems to be a more efficient platform for the 7900 GS. As a result, fans of either the ATI or Nvidia camp can be happy knowing that either two X1950 PROs or two 7900 GS' in dual-card configurations will yield very closely matched performance.
Without a viable $250 card available at the moment, dual-GPU setups in this price segment are looking good. Just be sure to read the fine print if you're interested.
Best PCI-E Card For ~$400
Radeon X1950 XTX
Codename: R580+, 90-nanometer technology
48 Pixel shaders, Eight Vertex shaders, 16 Texture units, 16 Raster Operator Units
256 bit external memory bus (512-bit internal ring bus)
650 MHz core, 1000 MHz DDR (2000 MHz effective) Memory
The fastest single GPU gaming card on the planet today, the Radeon X1950 XTX boasts much higher memory speeds than its X1900 XTX brother. Against the 7900 GTX in this price segment, its high memory speed gives it the performance edge.
Best PCI-E Card For ~$500
Geforce 7950 GX2
Codename: G71, 90-nanometer technology
2x24 Pixel shaders, 2x8 Vertex shaders, 2x24 Texture units, 2x16 Raster Operator Units
256 bit memory bus
500 MHz core, 600 MHz DDR (1200 MHz Effective) Memory
If you have money to burn, the money-is-no-object price segment will offer diminishing returns for the large amount of money spent, but if you need to play at the highest resolutions available with high levels of AA - SLI/Crossfire setups are the way to go.
Probably the most reasonably priced and easy to use multi-GPU solution is the Geforce 7950 Gx2, which is actually a single card with two 7900 GPUs built in, and it performs faster than any other single card available at ultra-high resolutions.
Keep in mind though that if you play at a 1280x1024 resolution with 4xAA, you would waste your money on the GX2, as cheaper single cards like the X1950 XTX can outperform it at those settings. But if you play at 1600x1200 with 6xAA, the 7950 GX2 can't be beat by any single card out there.
NOTE: Nvidia's Geforce 8800 GTS and GTX will be released soon and, while they may be more expensive, they will likely beat the 7950 GX2. You may want to hold off purchasing a GX2 until these new cards are released.