Does xbox live gold membership stack when I purchase a new one?

I currently have an Xbox Live Gold membership that will expire in March 2011. However the price of membership is increasing to $59.99 starting November 1, 2010.

If I buy a 12 month membership now at the current price of $49.99, will the additional 12 months be added on to my existing membership or will I start a new membership which will expire 12 months after I buy it?

Yes, they do. You can lock the lower price so when it auto-renews you save $20.

See here

One year subscription at $39.99 for the first year with auto-renewal at the then current annual price following year one

Yes, additional purchased membership time will stack. In addition to the one-year lock-in price currently offered by Microsoft, you can buy scratch cards at pretty much any retailer where games are sold. These can be bought at the current subscription price, and redeemed either now (and stacked with existing time) or sold to your friends later for a teeny tiny profit.