your kinda in the same boat ...
Here is what i call my Baseline for AH
this is what i get outta my Crucial 9600pro 128 card . this is 512 tex , 4.12 omega's ... this pic was taken today in the TA .

my friends 9800 pro 256 is alittle crisper , my x800 is in a 64 bit pc and its on the Hires settings and its a step up but runs the same fps ...
your not gonna get much more eye candy wise in lowres ...
so basically any 9800 pro , x 800 ,x850 gto ,1600 agp card is and should be at least as good and better then my 9600 card .
i believe if you want to see a huge improvement,,, its step up and go 64 bit .
I caught a 1600 pro 512 card in action the other day and it was a sweet step up ... but it was in a 64 bit pc and pritty equal to my x800 in "far cry"