I've always erred on the side of caution when it comes to wheels and bought XM or EX Mavic rims, but I never break them or bend them

how strong are the XC717 rims? what are their limits? what can't I ride on them?

Stronger than you'd expect for a XC rim!
I've just killed a 5 year old 717 that's had a very hard life. In that time its done 2 weeks every year doing lift assisted riding in the Alps, countless weekends here at various bike parks doing yet more lift assisted riding, tons of street riding, dirt jumping and the occasional XC ride! I only trashed it in a big off, although it has been looking a little tired the last few years.
You can ride whatever you like on them, but you do need to ride with a bit of finesse to make them last.
Very strong rims considering what they're intended for…

the newer named 519 may be more suitable and be better with wider tyres.

that's what I wanted to hear – I'd like some lighter wheels for the more XC type riding I do but don't want to be worried about them folding if I hit a couple of jumps here and there
I don't tend to break stuff anyway

good equal tension is worth a lot more than a strong rim ime.

juiced, are you sure it's 519? can't seem to find them.

the 717's are much stronger than you think. but if you're going to be particularly hard on them go up to 519's

As mentioned, stronger than you'd expect the 717s. I used to land 7 foot drop offs (to transition) on a hardtail on a CRC built pair and they never troubled me. Great rims!

719s are a much better rim and only 60g heavier.

If it needs to be light, use a ZTR Olympic. They give a better tyre profile and are lighter than a 717 by 60g. Not as strong as a 719 either.

2.25 tyres look really pinched on a 717 so you're limited to 2.1s and below. This also means high tyre pressures to avoid pinch flats.

A wider rim = wider tyres without an odd shape = lower pressures = more comfort and grip = 😀 😈

Simples

Excellent tough rim – tougher than the DT 4.1 or 4.2 in my experience. built properly they are tough, i'm yet to buckle one, dinged yes, but buckled no. Won't take a big tyre thou.

As Daffy says, might as well get 719s, which will stand up to 16 stone fatties like me downhilling.

did Les Gets on 717's with 2.35 High Rollers on them, and i'm not a light bloke, no problem at all.

most of the strength of the wheel is in the build.
the rim only really counts once you start dinging it through running stupidly soft tyres…. imo

got 2 pairs of 717s, only started denting them when they went on the full susser and i was clattering over rocks.

well my current wheels are XM719 disc on the rear and an XM321 on the front which you're saying won't be that much different and will have a better tyre profile for the 2.3s I normally use. So to get any benefit I should go for something even lighter??
Maybe I should go for lighter tyres first – currently using High Rollers (single ply 2.35 kevlar) which aren't that heavy – about 650g.

try en521 i think they weigh 510

Well they've been rolling nicely under my 16st ass for a few years now, with no problems & have stayed straight & true.

I've used a couple of Salsa Delgado rims in the last year or so after I had problems with fat tyres squirming arround on my 717s. 460g (claimed) and about twice the width of 717s plus added rarity value.

I ride a wide variety of terrain in the UK, and if you are aggresive or adventurous (especially in the lakes/rocky areas) I'd go for a wider rim. I only weigh 11.5st and I've killed 4 seperate 717 rims (3 dented and buckled multiple times, one crash) – I blamed the first two on them being 2nd hand, but they dent easily and are not very strong IMO. Main problem is they are very narrow, so you have to use <2.1 tyres and even with 2.1S, the tyre sits rounded and I always got pinch flats at less than 50psi!

this will effectively be a 'spare' wheelset for when I am just doing long XC rides where I will get the benefit from lighter wheels
I think my tyres are actually more like 850g so I think I'll change these first to something 500-600g and see if that makes a difference

I got LOADS more pinch flats in the days I rode 717's compared to 719 now. Same routes.

717 disc on rear, 717 rim brake on front
Panaracer Rampage 2.35 on both

I'm 95kg and I find them fine. I rely on the snake bite chamfer the Panaracers have, and you do get squirming at low PSIs but overall they are ok for me.

My rear has a flat side after a heavy landing at Afan. So they do dent easy, but that's about it.

I'd not want to run a tyre larger than a 2.3" on a Mavic 717, but I've used them and their earlier incarnation the 217 almost exclusively for the past 15 years.

They take tons of abuse and when made into a wheel by a good builder are bulletproof. Be careful when comparing Mavic rims though because only the top end rims are made using their Maxtal alloy, which seems to provide a good mix of low weight and good strength.

I've been using 719's on my other bike for the past eighteen months which I use for DH and general monkey business and they're a nice width for anything up to a 2.5" tyre. But a DT 5.1. is a little lighter , a little wider and just as strong.

That said I had a double pinch flat while riding through a boulder field and put a huge dent into the tyre hook section of one of the 719's. But the rim was still round and I straightened the ding with a pair of pliers and the rim has been fine ever since.

As chrishayes… 717s / Rampage 2.35 and at 25psi never a problem. I weigh 12 stone. Had 717s for years and their ? predecessers without any toruble!Stong and light.

… but not that much different to what I've already got

I use 2.3" tyres on 717's with no problems, they're very good rims, strong and light.
Mine have done loads of Lakes/Peaks rocky riding plus places like Spain, Alps, Pyrenees.