Now that the White Sandboxers have managed to accumulate enough gold to generate a substantial amount of XP, I'm going to officially adopt Arneson's beautiful interpretation of XP for treasure. I'll post some textual citations from First Fantasy Campaign later, but here's the general idea of how it works.
Each PC chooses something that matters to them. You earn XP when you successfully invest your GP in that passion of yours. The passion can be as broadly defined as you like, as long as it's clear what kinds of things would count as relevant investments. "Becoming a mighty king" is good because it's clear that you need to invest in gifts to nobles at court, purchasing lands, throwing feasts, demonstrating noble largesse to your henchmen, etc. "Living forever" is too broad to mean that you just want to invest in buying things that'll keep your character alive; in practice you'd forever be approached by alchemists claiming to have the elixir of youth, physicians wanting to become your personal dietician, and necromancers with proposals to make you a lich's phylactery.
It's definitely OK for you to try to invest in things that'll help your character succeed in the game, because the lack of a "magic mart" means there's no obvious way to do so; it's not like 3E or 4E where having too little money means your character is underpowered. You might decide that your passion is for collecting magic items, or treasure maps. Once in a while your investments here might yield something of use, but most of the time you'll just be paying agents to search after things for your collection, or acquiring minor curiosities with no immediately apparent utility. For the same reason, it's OK to invest in passions with no obvious game utility like getting as drunk, well-fed, and satyrically sated as possible. Sometimes your spending on these passions will also generate game opportunities like hearing rumors or meeting potential allies.
The major change I'm making from the traditional "wine, women, and song" XP rules is that the XP you gain for investing your treasure is in addition to the XP you gain from getting the treasure out of the dungeon. (I dunno yet if this is actually a change from what Arneson wrote, or just a valid interpretation that's different from the one most folks use). I think it's pulp appropriate that Fafhrd and the Mouser get XP for stealing the jewels from Thieves' House even if the heroes' larcenous paramours swap them for worthless paste as soon as they emerge. Instead of learning nothing from this unfortunate turn of events, they simply don't learn as much as if they were able to live in high style for months on the proceed, buying drinks for interesting strangers and alien fencing manuals at the Bazaar of the Bizarre. And for our game purposes, this will help PCs level in a satisfying way at our rate of play, and also help ameliorate problems where the PCs that are present to loot the treasure aren't the same ones who are around to spend it - this way we can split the treasure XP from each session among the players who were part of that session.
Down the road I'll make up random event charts for each kind of investment, similar to Jeff Rients' awesome carousing tables. For now, though, I want to get y'all thinking about your characters passions, asking me questions about how this works, and planning how you want to bring about these investments in the next session.
Note that the Keys from Shadow of Yesterday are also a potentially valid interpretation of Arneson's contribution to the OD&D experience rules.
