Fastest to load or fastest to respond? All Linux distro's load faster than
Windows, but not instantaneously - maybe a minute on average, depending on
how many "load on startups" you install. Ubuntu doesn't slow down after long
use the way Windows does. One of the fastest versions in terms of quick
response to commands is Puppy Linux because, while full featured, it keeps
the entire operating system in memory,

Phoronix (Google it) tested Ubuntu against Arch which is supposed to be
faster because you build it to your own specs and found little difference.
I suspect that your slow boot is probably due to a configuration problem.
Ubuntu 11.04 runs fast on my EeePC, so it is not likely a distro specific
problem.
I am a distro hopper and find Ubuntu as fast or faster than most. I also
have found that some distros run faster on some hardware for various
reasons.
There is no silver bullet unfortunately.
You need to test against your own hardware and needs to get a match. What
works for one user may not work at all for someone else.

I find Lubuntu faster than Ubuntu on slow hardware such as a netbook. However, I
can't stomach the file manager which comes with it. You have (more than) enough
memory to run Ubuntu, so you're OK.

I've tried both Lubuntu (LXDE) and Xubuntu (XFCE) which both use
faster, lightweight window managers but, as with gird, I didn't
especially like the file managers - much prefer Naulitus, but that's a
personal thing ...

If you're a gnome (2.x) fan, you may get on more with Xubuntu, as XFCE
is GTK based, same as gnome.

One point to take into consideration, if you're looking at Ubuntu
11.04, the full-blown install will default to the Unoty desktop,
rather than gnome 2.x ... you may need to install this seperately
(despite suggestions that it's still there as a fall-back - I've not
found this to be the case, and I was only upgrading, arther than
cold-installing!). A lot of new [gnome-based] distro's (such as
Fedora) are also now using the gnome 3 desktop, which differs
considerably from gnome 2 ... Ubuntu-based Linux-Mint, however,
retains the familiar gnome 2.x.

I've been happily running (full) Ubuntu on an Acer Aspire One netbook,
for quite a while now, without too many problems ... just so long as I
respect the hardware limitations, and don't try to run too many things
concurrently ...

You can run nautilus whenever you want, even when using Openbox or JWM.
Nautilus has a lot more functionality than Thunar, but, to my surprise,
PCManFM proved to have more functionality than Thunar. They were wise to use
PCManFm in LXDE. It is a good file manager.
Rox Filer is OK but I was lazy to configure it. You have to set a certain
file type to be opened by a certain program. In Puppy this fast little file
manager is preconfigured.

Yes - I've been considering using Nautilus within XFCE but, as you
say, too lazy to mess about with it ... Do you have any idea what the
underlying window manager is in XFCE? I know that Gnome (2.x) uses
Metacity (and has used Swordfish/Sawfish and Enlightenment in the
past), and that LXDE uses OpenBox (or is that BlackBox?), but don't
know what the deal is with XFCE ...

Is it just me, or should Zenwalk morally be using Enlightenment as a
window manager?

I think my biggest problem with the other file managers, is that I'm
simply comfortable with Nautilus ... when I ditched KDE (loved KDE3,
don't like KDE4), I was sad to lose Konqueror (you can keep Dolphin
... but that's probably for the same reason noted above), but
persevered with Nautilus until I got used to it as Konqueror is
unlikely to be as efficient in a non-native environment ... Never
particularly liked RoxFiler - even in Puppy ...