the 9800gt should work, if it is pci express and the motheroard has pci express, it should work
newer gpus that should work, yes, something like a gtx 480 more or less should work
a hd 7000 series perhaps should work too, but i mentioned the first two because i remember that is what was in use on that time, so it must work, also the power consumption and heat from a gtx 480 is considerably higher, so is better a bit older gpu
there are modern gpus that will work with the old bios from that motherboard, for example gt 1030, that should be 2 or 3 times more powerful than the 9800 gt and you can find it used, but i never used a so new gpu with a so old motherboard, is not impossible that you will find problems, so the first option are...