The 9800GX2 is about as powerful as a GTX 285, but I've heard of people having problems with them. The 5870 is almost as powerful as the GTX 295, but it uses a little less power and produces a little less heat. Its also alot cheaper so I dont see any reason not to get the 5870, except they're a bit hard to find right now.
The 295 is currently $85 more than the 5870 which means 4% more $$ on a $2000 build which compares with a 5-10% increase in performance....whether that's a worthwhile value to you only you can decide.
The 295 also can do PhysX....and gain, whether PhysX features are valuable to you, only you can decide: