GPS altitude should be very close (within say 50ft) to baro altitude if you have continuously set the correct QNH.
Just avoid units with the SIRF-2 chip, which have a constant ~ 200ft error, though all of those would now be quite old. SIRF-3 fixed this.
Memory Map is an OK solution for running a UK airspace map. It's no good for abroad; if you want a general European VFR moving map solution running proper VFR charts then you are looking at Oziexplorer and charts from bit-torrent
