I run 2x 7900GTX 512 in SLi just fine with everything maxed, no peass though.
But the original poster is running 9800pro, so he has an AGP mainboard, so the best upgrade without having to upgrade his mainboard is either Radeon X800XT / X850XT, maybe go for the Platinum models (PE), or the nVidia 7800GS which is the newest AGP card from nVidia and comparable to the Radeon X8x0's.
If you need anything better you will have to upgrade your mainboard also, but those cards i mentioned will run PE fine, also with 4xAA, as Entropia isnt that demanding on vidcards, it's prefers lots of RAM as the first thing, then vidcard, then CPU speed as third.