Sure is. Not PCI but L2 cache slot, which is why they are called 'L2 G3' upgrades. Also available with its own inbuilt 1MB cache (BG3-400-1M-KIT-04), as opposed to the 512kB in the linked example (BG3-400-5-KIT-04-A or E). I have four 5500/5400 with the upgrades, picked up when there was a spate of the cards on eBay in late 2005 and early 2006. The Macs (black) fly even in OS 9.1. USD20 is dirt cheap.
PS. If you max. the RAM (2 x 168-pin 64MB EDO 2k 70ns), replace the modem in the Comm. Slot with a CS II 10 or 10/100Base-T NIC, use the single PCI slot for a video card of better performance than the onboard video, and insert an AV card and a TV tuner, you'll be cooking with gas. (No. Not petrol).
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