Wattage for one slot is typically specified in the product details.
Wattage for one slot is typically specified in the product details.
The extra pins in the added versions aren't meant to supply power; they're only for bandwidth. The voltage remains the same as the original slot. Using a higher-speed adapter for a card drawing about 30 watts should be fine.
PCIe x1 slots are configured for a baseline of 10W, but they can handle up to 25W when not set to the full-height/high-power mode. The exact method isn’t clear from the link, but it involves adjusting settings to enable higher power without triggering that mode.
Your Optiplex using a 600W PSU should work fine on the board, though you might want to lower the voltage on the card in the slot.