I am not sure if the difference in performance between a 5800 and 5900 is worth the $100 dollars. They benchmark fairly close and both have their +/-.
Has DDR2 Memory (Faster) +
Has 128 bit bus (Slower) -
Has DDR Memory (Slower) -
Has 256 bit bus (Faster) +
Overall the 5900 is quicker, but not sure if its $100 dollars quicker.
As with anything in computers, if you wait the price will come down. If you want to buy now and are on a budget, the 5800 would be good reasonable choice. If money is no object, then the 5900.
Hope this helps a little. Ultimately its your decision. Hopefully others will post their opinions on your choices. Whatever you do decide, enjoy your new video card