I have attached an Excel sheet that I made with the equations needed to design a barometric altitude sensor.
For lower altitudes and for small altitude deltas, one can assume that there is a linear relationship between pressure and altitude. Please see the attached Excel sheet for more details.
The SCP1000 pressure sensor seems like a very good candidate for a pressure sensor:
The manufacturer gives these data:
Resolution: 3 Pascal (the fineness to which the sensor can be read - nothing to do do with the accuracy)
Relative accuracy (how close to the real pressure the sensor measurements are) over temp range +10C to +40C and 600 hPa to 1200 hPa: +/- 50 Pa. (It is not given in the datasheet if this is sensor to sensor or for the same sensor)
The manufacturer does not give noise data. However sources on the Internet may indicate that the sensor has an inherent noise of +/- 50 Pa.
Sample code: http://www.sparkfun.com/datasheets/Components/SCP1000-D01.pdf
Another guy using the sensor for a variometer for paragliding: http://www.pixelproc.net/varios.html
The noise may be filtered out using an averaging filter however. Even a kalman filter and two sensors may be used.