Is this PCI graphics card intended to be a secondary card, used in conjunction with the primary AGP graphics card in your G5 or did you pull the AGP card? If the original AGP card was removed, you'd be much better off replacing it with an AGP 8x-compatible card, because the AGP bus has much more bandwidth than the PCI bus. Additionally, the graphics card must be programmed with a Mac ROM to function in your G5. If the Sapphire card has already had its ROM chip flashed with a Mac ROM, it should be detected and function normally. Otherwise, you'll need to do it yourself. There are web sites dedicated to this, so a search will find the instructions. Over 10 years ago, I flashed about (8) ATI (retail-boxed) Radeon 7000 PCI graphics cards for use in my older G3s, although the procedure was done in one of my PCs. The Sapphire Radeon card is referred to as "Powered by ATI," whereas ATI's own retail products are "Built by ATI." When flashing certain third-party "Powered by ATI" cards, some have smaller ROM chips and require a slightly truncated ROM file. Again, the web sites dedicated to flashing graphics cards for use in Macs will have the specifics.