I would recommend you check out the site "Stuck in Customs" (stuckincustoms.com). It is a site run by professional photographer Trey Ratcliff. On the site go to the tab "Stuff you need". He has quite a few good recommendations - and some fabulous HDR photos that you might enjoy.
I use the Manfrotto 190CXPRO4 when traveling by air as it just fits into my suitcase. However, my heaviest lens is only about half the weight of the Sigma 150-500. Since you will be mountaineering, wind is a considerable factor to consider. Although it is easy to carry and folds up nicely, I think you might want something a little heavier and stronger than that particular Manfrotto. Right now I think some of the best bang for the buck in carbon fiber is with some of the Chinese brands (there are several that seem pretty good). With those you should be able to find something that is closer to the strength of a Gitzo, but closer to the price of a Manfrotto.
I'd say your best bang for the money is to get a Benro Combination series tripod [link] this tripod can even support the 800mm lenses with ease....so it's great if you plan on adding larger lenses in the future. That said, if your biggest lens is gonna be the 150-500, the classic carbon series from Benro would be more than enough for your needs [link] As for a head, you'd need a gimbal head at that focal length, and best gimbal heads are made by Wimberley. Although, if you don't plan on getting bigger lenses anytime soon, I'd get a ballhead (Benro ballheads are good, but the best ballheads are by far made by Markins), and the Sidekick from Wimberley (it attaches to your ballhead making it into a sort of gimbal head). If you don't need to track wildlife and birds and such, the ballhead alone would be enough.
I'll most likely get flamed for this, but RRS tripods are overpriced, and mainly you're paying for the whole "made in USA" thing. Gitzo tripods are good, but the Benro are just as good (even better in some cases), and are cheaper in price. Manfrotto tripods I never used, so don't really know much about them. I do use their lightstands though, and they're great.
Manfrotto has gone down a bit in terms of quality. I have one of their low-end tripods and it’s been gradually disintegrating on me over the past two or three years. Their carbon fibre tripods might still be all right, but they’ve definitely cheaped out on the “pro-sumer” level.