IMO The X1900XT/X is the best single card GPU out in the market, you can get a HIS X1900XT with the IceQ3 cooler for $420 dollars after a 10 dollar mail-in rebate on Newegg. People still keep repeating (like what nubius said) that ATI's drivers sucks, this is very true for the X series of cards, but not true for the X1K series. While Nvidia's drivers may still be somewhat better, it's like saying since the 60,000 dollar car is better then the 50,000 dollar car, that the $50k car sucks. This is also why people buy the best of the best and are spending too much money on it, just look at SLI, or, god, even quad SLI. ATI's X1K drivers are pretty good, a huge step up from the X series.
People keep recommending Opterons because everybody else recommended it, I know this is true because someone actually said "I was following the crowd" or something of that extent.
The X1900GTO isn't a bad card except it costs more then the 7900GT and ATI really screwed up with the card. They locked 12 of the 48 shader pipelines and underclocked standard X1900 clocks, maybe they didn't want people buying the X1900GTO and overclocking it to XT/X speeds but they should of kept the shader pipelines as that is the thing that puts the X1900XTX next to the 7900GTX.
I remember when people kept saying they are not going to overclock but people say "ooh, you will overclock, you may think you won't but you will, get the Opteron" even though the 3800+ is better (and cheaper) then a stock Opteron 165. That's almost as bad as when Green Radience told me (and others) saying things like "he won't do this, or he won't do that" YOU ARE NOT HIM! DON'T SAY WHAT THEY WANT OR WILL DO BECAUSE YOU ARE NOT THEM!
I would just go with SM3.0, either get the X1800GTO or the 7600GT, either will make you happy (and they both have SM3.0).
X1K drivers don't suck
The X1800GTO can do HDR+AA, AAA, and HQAF
The X1800GTO handles AA/AF great like many ATI cards do (a X1800GTO runs at almost the exact same speed as the 7600GT while playing Doom 3 w/ AA/AF)
But the 7600GT is a good card and plays FEAR better then the GTO, but that isn't quite as importaint as having better frames in Oblivion while being outside (where framesrates can easily go to the unplayable level).