I know about all the newer cards but I need advice on these two only. Which is the better choice, ATI X1800 XL or Nvidia geforce 7800 GT? Or would dual 6600 GT's be better? Thanks for any help!

My advice would be to wait for the 7900GTX whatever the card might be you will buy.

They will prolly drop in prices all of them.

My personal preference however would go out for the GeForce 7800GT /GTX

7800gt
read this
http://www20.tomshardware.com/2005/12/02/vga_charts_viii/page5.html

7900GT out next week or so for same price as 7800GT

One can now (today) get the X1900XTX for $499 and the X1900XT for $449 at Newegg. I imagine prices will fall a bit more after the G7900 is on the market.

Rumors put the G7900 at 10 percent faster than the G7800. If so the G7900 will win some (older benchmarks) and lose some (newer games) against the comparable X1900.

Depends on how long you plan on using/keeping the new graphics board. If it has to last you this year and a big chunk of next, I'd strongly recommend a X1800/1900 for their undeniable branching performance lead. If you're a more frequent upgrader, then you really won't go "wrong" with either brand.