The general recommendation for streaming video is 2mb/s. My isp provides 2mb/s upload and 50mb/s download.
However, it is difficult to give you a solid answer to your problem, because you may be experiencing wifi interference from neighboring wifi systems, or even a problem with your wifi router.
I stream You Tube and Netfix videos via wifi to a blu ray player with very good results here. BUT, I check my wi fi environment with the built in capabilities of my iMac to make sure that I'm operating on a channel that is not being used by the other 9 wi fi networks in my area. My Wi fi system is a year old "n" capable Air Express. (pretty fast)
My gut feeling is that your problem relates #1: most likely wi fi interference, #2: less likely, your 1.5 mbs/s isp speed, #3: least likely, your wi fi system.
Sorry that I can't be more definitive, but streaming video via wifi can be a dicey proposition.
So is 6 mbps enough to stop the buffering problem?
Unfortunately, no one can tell you how a wireless signal will perform in your home unless we visit your home.
Your chances of smooth operation improve significantly by going to a 6 Mbps speed, but there are always a number of variables with a wireless signal.....distance, obstructions, inteference from other sources, etc that affect the experience.
Personally, I find that I need at least a 10-12 Mbps connection for solid, reliable performance from the Internet.
The bottom line is that you will not know if 6 Mbps will provide the improvement that you seek until you try it out in your home. For that reason, you might want to make sure that you understand the terms of the agreement before you commit.
For example, there might a "trial period" to allow you try things out before you move permanently to the higher speed plan.