One of my favorite sourcebooks from the old OGL days was, and remains, Relics & Rituals: Excalibur, from White Wolf's short-lived D20 division. In a way similar to Green Ronin's Mystic Vistas series, this sourcebook details how to use the d20 system (specifically D&D) to simulate an Arthurian-style campaign that is both true to the feel of Arthurian Myth, while preserving as much of the Dungeon Fantasy gameplay as possible.
This is a 3.5 book that's easily adaptable to Pathfinder, as most of the more useful content is fluff for roleplaying an Arthurian-flavor campaign, like which Classes fit the setting best, setting-appropriate racial write-ups, which monsters fit an Arthurian theme, and so on. It has some nice twists to classic D&D, like combining the three Goblinoid Races into just "goblins", which come in various "sub-breeds" like how mastiffs and terriers are breeds of dogs. In Excalibur, the term "Hobgoblin" is the common name of the Anhardd, which are to Goblins as Half-Orcs are to Orcs.
One chapter also deals with the topic of religion, especially the delicate topic of Monotheism and how Arthurian settings typically have an "Old Gods vs New Religion" conflict, and how to handle it maturely. The sample "Excalibur" faiths presented are the Daeosian Church, a monotheistic faith with elements of dualism, Druena, a druidic "Mother Goddess" faith, and a set of pagan deities for the Fae, which fit a celtic theme more closely.
The book includes a Knight class that's easily replaceable with the Cavalier, and all of the Advanced Classes (save the Gunslinger) would fit an Arthurian setting rather well. Witches fit Arthurian lore just fine, Oracles make good "Holy Hermits". The Book's Prestige Classes should be easy enough to Convert, and most are flavorful enough to warrant it, like the Fey Enchantress, the Green Knight, the Malefic Enchanter and the Reliquarian.
One particularly fun optional rule in the book are the Honor rules, which allows you to track a character's Honor via their behavior and adherance to chivalrous virtues. This is aided by chivalric Orders and Codes of Chivalry (as opposed to Cavalier Orders), with two interesting variants: the Dark Warrior's Code for black knights and other villainous warriors, and the Order of the Gold Octagon, an order of "White Wizards".
All in all, it's a product I recommend. It has a lot of good, useful advice on how to handle an Arthurian-themed campaign.