Will a Gtx 460 fit in a gateway sx2803?
I don't believe so. The sx2803 although it has a PCI Expressx16 slot, the specs indicate that the replacement graphics card needs to be a half-height card. I think you would be hard pressed to find a 460 in half height, assuming that one exists. Also, the power supply in this machine would also be a limiting factor, it doesn't appear to be a standard size and therefore you would also be limited in sourcing an appropriate power supply that could power a higher spec graphics card.