Technically possible with additional BIOS modding. Practically useless for such weak hardware.

Yes, 512MB is as much as a HD3850 can reasonably use. Wherever you'd get any meaningful (in percentage at least) performance increase, the GPU would be too slow to output playable frame rates. It's not worth risking an uncommon late-AGP graphics card.

It could be done , but you have to keep in mind what you gain . On newer gpus for high res gaming it would make sense but on old ones , not all the time . Ex : in the experiment i did on a FX5200 (going from 128mb to 256mb) i'm loosing about 550 points in 3d mark 2001 (going from 128 to 256) . I didn't had to mod the bios for the memory size , all was done by moving resistor jumpers so i think in most cases , If you don't put the bios from another gpu pcb implementation / gpu model , it can be done at a hardware level only . On old video cards for the best performance gain , you have to keep in mind what type of games you play and how much VRAM they require because of the benefits of having more VRAM / the computing power lost to address more VRAM . (I'm keeping my FX5200 on 128Mb even if it has 256Mb now and i did more because of the faster speed memory IC's) . So Yes if your doing it with memory overclocking in mind / No for only the size increase .