9800 XT will still outperform the 5950 Ultra, despite the 5950 Ultra's higher potential memory bandwidth. Certain games, however, seem to favor GeForce FX cards more than the Radeon 9x00, such as Splinter Cell and Doom III (though that may change by the time the actual game is released).
ATI! The 9800 XT is much faster than the 5950. Plus, the benchmarks Valve released showed the 9800 as being twice as fast as the 5950 for HL2. Go with ATI.