I do not see any reason why you wouldn't be able to install that GPU. It goes without saying that if you have an AGP motherboard you can only use AGP graphics cards. But the other main things to check are:
Physical space - if a card is 30 cm long, and you only have 25 cm space at the card slot, you're not going to be able to install it (at least without modifications to the case).
Power requirements - Check card manufacturers recommendations for minimum power supply requirements. High performance cards (for their time) may require the typical 250w-350w PSU's supplied in most pre-built desktop PC's to be upgraded.
AGP voltage and version (particularly with very old hardware) - See this article on AGP versions - http://www.hardwaresecrets.com/article/AGP-Bus-Tutorial/155/3 Basically cards with multiple "keys" should be able to go in any slot, but an old card keyed for 3.3v won't go in a 1.5v slot.
This PC/GPU will be mining digital currency, if that helps. Thank you
From what I have seen of "rigs" used for this purpose, I don't think so. Custom hardware (ASIC) for mining is now available and is orders of magnitude faster than even the most high end modern GPU's (and anything that goes in an AGP slot is at least 5 years old to start with). l honestly think you are wasting your time.
See comparison of hardware here (but note comments about ASICs at the top) - https://en.bitcoin.it/wiki/Non-specialized_hardware_comparison