There are a few authorised Yamaha dealers within 20 miles of my house, so I will be taking a few trips this weekend to see what's available. I'm going to try to avoid getting it off of the internet.
I have just a couple more quick questions... so I know what to expect when I go searching.
Are the prices for digital keyboards set in stone?
How closely would a piano like the P90 behave like a normal piano?
How long do they generally last, if they are well taken care of?
Are the harpischord/organ/effects just for show or are they actually usable?
Prices are pretty much set in stone, unless you can get a floor-model. Internet dealers are cheaper as they don't charge taxes and often not even for shipping. Sometimes, you can get local dealers to match Internet prices, but you'll still have to pay taxes.
No action on digitals will be like a real action. The Yamahas have an excellent, albeit a bit heavy, action. Typically for Yamaha, the keys are rather deep, much deeper than, say, regular Steinways. I personally like the Yamaha action better than any other action on reasonably priced digitals, but that's clearly personal preference. The bottom line is that the Yamaha action on the P and PF-series is very good. Keep in mind that digitals usually don't simulate the escapement mechanism, only the high-end ones do (several thousand $$).
I must repeat myself: you must decide for yourself how well a digital piano resembles a real piano. Play them all, even those that are well outside your budget, so that you can see what the span is. It helps if the store has acoustic pianos as well to compare. All digitals have demo songs that you can play with the press of a button, so that you can hear what the instrument is capable of. You can change voice, tempo, etc. on the pieces, too.
I believe digitals from the reputable manufacturers last very long, even if you occasionally play FF, which would have worn out older digitals rather quickly. Remember, the stage pianos that most pop and Jazz musicians use are exactly what you are looking at.
Depending on the model you choose, the sounds will be anywhere from awful to excellent. The P-series Yamahas have good samples. Definitely check them out with headphones, as it's often noisy in stores.
I personally have a PF-500 for practizing in piece and quiet. Far from other people's opinion, I don't think it sucks

Finally, however many digitals you check out and whatever the salespeople tell you, do not buy this weekend. Write down which models you are considering, download (and study!) the manuals, check the Internet for reviews (particularly on the sites I listed in an earlier post). If you still have questions, ask again.
Have fun!