The quotes by bro SS08 are correct - take the bus and book only when you arrived in HCMC and head to De Tham Street, which is near to Pham Ngu Lao in D1 - abundance of tour shops over there and once again, book whole package e.g. hotel...much cheaper and won't be a rip-off...
Oh bro vitruvius79, if you and your gal ever do some "exercises" on board the train or bus, do let us know which bus or train you took, so that perhaps bro SS08 and me can avoid taking lest we see some leftover evidence on the seats... :P
Nha Trang is a long bus journey - ideal to take a night bus with sleeping bus: proper bed....i took the day trip and end up having to go for a massage right upon arrival in Nha Trang and right upon my return to my apartment in HCMC. No joke...and besides, the locals love to chit chat on the bus which makes for one long and horrible bus journey when you are trying to get some sleep.
Enjoy!!!