I'd be careful in relation to the new 840 drives. Not sure if i'd be happy with putting a brand new drive just out of development into my system - you don't really know how reliable it's going to be.
Samsung 830 prices have been going down recently, and it's still one of the most reliable drive on the market, whilst being one of the fastest in real life scenarios. If you can bag a cheap 830, i'd go for that instead.
Just my take on things.