Good article here in the USofA (AOPA mag) regarding true GPS and PDA "add-ons".
I was thinking of an iPaq plus the goodies (will cost same as a Garmin 196) but read that you WILL experience a lock up or two. Plus, batteries will get used, and there are a few wires/anntenae to consider.
Still, definately the most flexible. I have even seen an HSI add on for a PDA.