I have a Radeon HD2600XT GDDR4 overclocked to 850 core/1170 memory (2340 DDR). I have never seen this card hit over 65 C. This is on a Crossfire 480X motherboard with an AMD Athlon X2 3800+ running at 2.56 ghz and 2 GB of memory running at 426 mhz (852 DDR). The video card is currently running at x16 bandwidth due to the fact that only one graphics card is installed. I score roughly 11,000 in 3dmark05 and get an average of 400 fps in ATItool 3d view. I also get constant 20-40 fps in Crysis on medium settings at 1024x768.
1. Would adding another 2600XT GDDR4 be worth the money?
2. Would I see a large jump in performance?
3. Would the slower x8 lanes slow things down noticeably over an x16 system (would a 580x or 3200 chipset be worth upgrading to)?
The 2900Pro 256-bit card is still a great deal. The 256-bit bus doesn't seem to affect performance at all. Just keep in mind that the 3870/3850 and the 8800GT/GTS G92 use a 256-bit bus. It is extremely overclockable. I have mine at 850/1000 and I get 11k in 3Dmark06. It's a little loud but for the performance and price you can't beat it.
I run Crysis with it using the DX9 Very High mod and I get about 30FPS @1280x720. I highly recommend this card to anyone that doesn't have a $10 dollar PSU. The power requirements aren't as high as the old Pro or XT because it only needs one 6-pin or 8-pin adapter to run it. They are going on Newegg for $159 right now, I was lucky and picked mine up for $130.