Thanks everyone for their advice!
I've successfully upgraded to 10.6.5, but not without some trouble along the way.
First off, something is wrong with the Combo Update from Apple. Every time I try to download it I get the message "Mounting Failed."
I ended up downloading just the regular Update from Apple's support page. Then I ran the dmg and installed everything. Before I restarted I used Cindori's Zeus and downloaded Netkas's QECI patch for the 4890.
Then I restarted. Immediately after hitting the restart button I got a blue screen. It didn't seem like the computer restarted or was installing any updates. The screen just hung in that state. I let it stay like that for about 10 minutes. After that it was pretty clear to me that something was wrong.
I hard shut the computer and pressed the power button. It started, I heard the chime, and took a few moment before showing me another blue screen where it hung for about two minutes. This time it sounded like the computer was doing something and the hard drives were spinning up. Then it got to the screen with the Snow Leopard galaxies (the same screen you get at the login window). Once it got there I still couldn't log in, however. A new window popped up and said an additional installation was needed, and it gave me two options: "Restart" or "Install and Restart."
I chose "Install and Restart." Another message immediately popped up saying there was an error and I had to restart. I restarted. This time everything went fine all the way to the log in screen. I logged in.
Apparently now I'm running 10.6.5. Everything else seems to be running fine as well. No issues thus far.
The installation of 10.6.5 was abnormal to put it mildly. My question is, should I be alarmed over anything because of the way it went?