819's are VERY strong rims for their size. Bit narrow mind, wouldn't run anything over a 2.3 on them as it would give a rubbish profile and be likely to roll too much.
They are a bit weighty for an XC rim too (weighed a naked rim with the 32 eyelets once and it was 510g all up), but they are immediately tubeless ready without any modifications which wins them some bonus points back.
Stans rims are the obvious competition, and which to go for would depend on preference. 355's would be the lightweight choice (Olympics are just gonna be too light for anything except XC racing I fear), Arch's the more robust and the Flow's for a stronger/wider rim still. They're not immediately tubeless ready though (need a rimstrip to convert them for use with normal tyres, or a valve and the yellow tape with UST tyres). They're also even more expensive than the already expensive Mavic UST rims!
Hub choice, well what do you want to spend? Hope Pro2's have to win out on the best combination of cheap/light/strong. Of course there are better hubs, but you pay a lot more for them! If you don't mind running centrelock rotors and have a QR fork, take a look at Shimano XT hubs too. Significantly cheaper than Hope, only marginally heavier, as durable if not more so, and plenty tough enough. Or XTR, or even DT Swiss 240's if you're feeling flush!
Personally though, having tried to have a set of wheels that's capable of being both XC race light, and tough enough for messing about on, it's pretty much nigh on impossible. I'd get 2 wheelsets myself. Probably a cheap pair of Deore/SLX/XT hubs on say Mavic EN521 rims for messing about on, and go with some Hope Pro2's on Stan's 355's for XC racing etc.
Oh, and spoke wise on whichever wheels, go for DT Comp with brass nipples. Anything lighter/thinner than a DT comp is pretty flexible in my experience, and expensive too these days! You can save 50g elsewhere from the bike much easier than on your spokes… And Brass nipples cos they're much easier to re-true… Ally nipples round off too easily.