2004 Sienna LE - from 16 to 17 inch wheels?

If you get the correct Toyota (or equivalent) wheels, you shouldn't have to do anything. (Of course, you'll have to buy tires)
The only warning is that the stock 17 inch alloys from 2004 to 2006 are known to get bubbles in the paint.

If I were you, I'd go with 16 inch alloys. My van has them and they are pretty good.

You could try www.tirerack.com. At this website, you can select your vehicle and color, it will display your vehicle. It will show you all the different aftermarket rim options and sizes. You can click on any rim and it will display it on the vehicle. Gives you a feel for what it would look like, Kinda cool. They will mount tires and ship directly to you ready to install. If you have the tire pressure monitoring system you may have to reset this. I had to reset this after I replaced my tires and after i had a flat repaired.

I really appreciate your comments...this forum is great! I just purchased a Toyota Certified 2004 with 56,000. It came with the standard non alloy rims (I really don't like them). The reason that I ask is that I have the opportunity of purchasing a used set of 2004 Toyota OEM alloy 17 inch rims. I haven't been able to find the 16 inch alloys for a good price. AS much as I would hate to let the 17's go, I'll probably wait until the 16's come along.

Buy them and use AVID TRZ's. Your handling will be much improved.

I'll second the Yokohoma's. I replaced my run-flats on the '04 at 30k miles with the Avid TRZ. Handling and road noise were significantly improved.

ours came with the standard 16s and i purchased a set of alloy 16s on ebay from another toyota. i wanted to stay in the same size so i could swap tires and also i use the steelies as a winter set

The 16 inch tire and the 17 tire have different profiles.

. . . cutting a long story short,

the total wheel will be the same size regardless of whether you are using a 16 or 17 rim assuming you are using Toyota approved tire sizes.

So no problem at all except for the cost.

It looks like the same discussion is currently going on in three threads.
Difference in wheel diameter is already discussed here:

http://siennachat.com/forum/index.php/topic,686.msg4902.html#msg4902

http://siennachat.com/forum/index.php/topic,695.msg4951.html#msg4951