First things first: I AM NOT AN ELECTRICIAN!!! Take my advice at your own risk.
Regarding three wire plugs in two wire circuits, there is a way to ground these circuits under certain circumstances:
1. Your house is wired with armored cable (the wires are wrapped in a metal sheath) and you have metal switch and outlet boxes.
2. For each circuit (run of wiring to the main panel), you can verify that the cable's metal sheath is intact through the entire run (no plastic boxes in the circuit, say from a past splice for a new outlet). This is very important!! It must be intact all the way to the main panel.
If you have these circumstances, you can install a short run of wire (green insulation or bare copper only, please!) from the green screw on the outlet or switch and screw it into the box. There are usually holes in the boxes for these types of connections. They sell these wires (ask for "pigtails") at most big box or regular hardware stores.
By doing this, you can provide a true ground for each outlet instead of creating a dangerous illusion of a home full of grounded outlets and switches. Naturally, the best way is to provide a true ground for each circuit directly to the panel, but this is definitely better than nothing.
Finally, if you go with a GFCI outlet and don't have a ground attached, you should find a sticker in the box that says "No Equipment Ground." That sticker needs to be put on the outside of the outlet.
Good Luck and don't kill yourself! ;-)