IIRC you can replace the chips with faster version of the sizes it already has, increasing the size to say 128Mb would require knowing which caps on the PCB to change the Vram straps, this is assuming that the PCB has the support for larger Vram to begin with, it could also require some VBIOS fiddling to get it working.

Just plonking on bigger Vram ICs likely wont work on its own.

Also, I dont believe the GF3 Ti 500 came with any other memory configs out side of the 64mb versions so that would also be an issue as there wont be any other memory straps to work with, unless there was a weird AIB/3rd party board with more and you can source that model for its bios and pics of its PCB design.

I was wrong there is a 128mb version just that its super rare to see one on eBay or anywhere else, if you can find the 128mb version of your model.. you could work from pics of that to mod yours, pics of the PCB and Vram ICs would help a lot here.