The Orinoco is the right way to go. It's a high quality card that has a good warranty to back it up. Also, if you plan on running OS X (using ex post facto) you could also run a card that has a Broadcom chipset. Unfortunatly, there are not too many cards that run those.

Another that will work with your machine is a Buffalo Technologies card. From personal experience the Buffalo cards are crap. Yeah, I wouldn't endorse them with a ten foot WiFi pole. But if you want to, that is cheap card you can try on your PB.

Now for your WiFI issue. After updating to OS 8.6, you will get internet access, but it's not going to be very fast. It's going to act like if you plugged a NIC into it. Pretty much, your limited by your machines speed. So if going online is high priority, then get a card. If not, go with one of the mods.

The 1400 makes a good picture frame with the right software. They are not too hard to modify, considering all you will need are some Torx screwdrivers to take apart. The carputer, I'm sorry, would not be a good idea. It will just run slow, and not be too impressive. A good carputer, in my opinion, should be atleast a G3. With the speed, and processor, you can run OS X where there are some *nix apps made that would look great in a car.

I'd go for a picture frame that can play MP3's.
MacBook 2GHz Intel Core 2 Duo w/ 2GB DDR2 RAM & 120GB SATA 5400RPM HDD
Canon Rebel XTI
Google Cr-48 Beta Laptop