Well.. To be 100% honest if I were given a choice to pick a GTX 512 or a X1900XTX, I'd pick a GTX 512 without a second thought. Don't get me wrong I believe the X1900XTX is faster than GTX 512 in general and ATI's image quality is great. I've owned many of ATI cards in the past.
However, there is something other than just speed.
1. ATI really need to hire some industrial designers.. Their boards, coolers, stickers, even the logos and advertisements.. ugh.. why can't they hire some top-notch designers? They sure can afford to do so, I believe.
2. I love the GTX 512 cooler. It keeps the card very cool (My max never goes above 60C) and extremely quiet. One more thing I can actually clean the dust off the fans! Believe it or not it's a big deal for me. I don't think it'd be easy to clean the fans on X1900s. Lower GPU temps = lower system temps = better CPU OC
3. Have you tried OC GTX 512s? It goes easily above 625/1800. Not sure how it'd compare to X1900XTX stock, but I have a feeling that it'll be close. Of course I have no idea how XTX will OC, but given the information on its temps, I wouldn't want to OC much higher than its stock speed.
4. I still have some reservation on ATI's drivers. You just can't predict where the quirks will show up. (Age of Empires 3, Black and White 2, etc.) System level compatibility also seems sometimes questionable. (Especially with those AIW cards) It could be a result of NV's marketing (TWIMTBP) or incompetency of ATI drivers team, but regardless I just don't want to have such occasions at all. I was extremely temped to get an R520 cards when it came out, but I'm so relieved that I didn't buy one. I'm currently playing Age of Empires.
5. I wanted dual card setup. The choice is quite obvious. Does anyone own a cross-fire motherboard?
Just my thought.