I doubt that an AGP to PCI to PCIe adapter will get you anywhere with a Pegasos2 speedwise. AFAIR, the Pegasos2 only gives you a PCI 66MHz slot with an AGP connector, that means that non of the additional AGP DMA transfers really work which are important to get any of the more recent (AGP) cards up to speed - if graphics acceleration works at all without proper DMA, that is ...
Even if you can get any newer card to work somehow, don't expect that you reach any better performance from it which makes the whole setup kind of pointless IMHO.