I just installed openSUSE with KDE (because I used KDE 3.5 before and liked it), but 4.3 just seems too bloated and eye-candy centric. I wonder if I can install GNOME and be able to choose GNOME or KDE on startup?

I also chose the option to login automatically, so I have no login prompt.

Well, I know you can go into OpenSUSE's package manager and install GNOME from there (should be "gnome-desktop" or something to that effect, a few hundred packages, should be pretty straight forward).

Typically, the login screen is where you would select which desktop you want to use (under "Options"), but I don't know how it works with automatic logins. I guess you can change that option to give you a login prompt, but I don't know for sure.

Beside installing GNOME using yast you also have to disable the automatically login.
Because you have to choice you're GUI at the login screen .

just start yast / software and install the pattern for gnome

log-out of kde and you will see gnome in the menu (bottom left)

With more than one...

As mentioned, at the boot splash, while you watch the gecko and the disappearing circle of dots, look to the lower left and right click your mouse opening a menu to choose between desktops.

If I remember, the system will default to boot the desktop that it last booted unless you choose differently.

I too used KDE 3.5 with pleasure and found KDE 4 foreign. Then I remembered that it corresponds to the problem of all those habituated only to Windows using linux for the first time; that first experience isn't alway a fair measure of the end quality.