It will be much better than your 5600.
The 5900 is a great midrange card with tons of raw power. Not sure if I'd pick one over a 9600XT due to the image quality issues and relatively weak shaders of the 5900, but the 5900 has enough raw fillrate power that the cards are neck and neck in my books. You can't go wrong with either card.
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<b>Radeon <font color=red>9500 PRO</b></font color=red> <i>(hardmodded 9500, o/c 322/322)</i>
<b>AthlonXP <font color=red>2600+</b></font color=red> <i>(o/c 2400+ w/143Mhz fsb)</i>
<b>3dMark03: <font color=red>4,055</b></font color=red>