Resolution of photo paper?

What's the resolution of photo paper? Googling for terms
such as dpi and noritsu, I came across marketing claims
from minilab manufacturers claiming 400 and 500 dpi. Is the
the current bottleneck the paper or the equipment used to
project light onto the paper?

I just bought a 10D. I am going to print primarily 4x6's and
I've decided to scale down the raw image to 75% (2304x1536)
to generate jpegs suitable for printing at 4x6. This gets
me 384 dpi.

I found that when printing the same G3 photo at 4x6 and 5x7
(378 dpi vs. 324 dpi) that I could discern the difference in
detail and the the 4x6 had a noticably more tack-sharp
quality, but this could be due to the sharpening settings
being more optimal for the 4x6 print.

Here are some of the first images from my totally digital
10D (not to be confused with the totally obsolete D60 ).
I'm using Breezebrowser with quality factor 99. This keeps
all my images under walmart.com's 3MB upload limit.

http://w3.kill-9.com/batterypark.jpg
http://w3.kill-9.com/liberty.jpg
http://w3.kill-9.com/relative.jpg

Andrew