I would recommend a DID ERT 520 series chain. O-rings are more expensive, and on a 125, they do rob you of about one HP or so (so others might atest to this). If you do a lot of trail riding then an O-ring chain might be good. They do last longer, and you don't have to tighten your chain as much. But as they get old, they tend to kink. And you always have to make sure you care and clean for it properly, like using "O-ring safe" lubricants. Another tip, you should always replace your chain and sprockets together in complete sets. It's the only way you'll get the longest life out of your chain and sprockets. Putting a new chain on old sprockets will lead to premature wear. I have no experience with X-ring chains, so I can't comment there, but they are about just as expensive as an O-ring if not more.
For tires, I would recommend a good intermediate tire like a Dunlop D756, it's just a good all around tire. But I hear a another tire like the Maxxis IT, is similar in design but is supposed to last longer than the Dunlops.
I have to agree with most of what Yogurt said. However, on the changing our the chain and sprockets as a set...
You don't really need to change out your sprockets IF you have kept a good eye on your chain and know when it gets to no more than 1-1.5% elongated from new. (more like 1% on a ring chain)
When a chain does get past this measurment it will start to damage the teeth of your sprocket. This deforming of the teeth is visibly noticible aftrer awhile, but the initial change in seat of the sprocket will be hard to see right off. The only way to know when you approaching this damage is to keep an eye on your chain measurement.
I always recommend that a rider uses a standard type chain on ANY bike, then have a ring chain on hand to change out when you ride in the slop. When done with riding in slop, change back to your standard chain. You can get a few years of service out of two chains and one set of sprockets that way, IF you practice good chain/sprocket maintenance.
Btw...I mentioned above that it is more like 1% for a ring chain because when a ring chain starts to elongate, it is only going to be a very short period before it throws craps on you and ruins your sprockets. The time period of a standard chain going from 1% to 1.5% is FAR longer than a ring chain going from 1% to 1.5% for the simple fact that you cannot lube the ring chain properly once the internal lube has been consumed.