My sound problem was fixed by turning the volume up (couldn't feel any stupider if I'd forgotten to check if the computer was plugged in!).
Kext Helper crashed exactly the same way as OXS86Tools and Kext Utility before it, with the same console message about a file named MultitouchSupport being of unrecognized type. I copied the MultitouchSupport file from the install disc over the one that was installed and, there you go, all three programs now run fine. Like I guessed, the file must have been corrupted during installation.
Found that OSX86Tools was essential for getting my screen resolution to work right. Just putting a "Graphics Mode" string of "1680x1050x32" in com.apple.Boot.plist only resulted in rebooting to a max resolution of 1280x1024 instead of 1024x768. In order to get the optimal resolution from my graphics card, I had to generate a humungous device-properties section in that file, as well, by choosing my video card from the option in OSX86 Tools.
Ran all the permission-fixing and clean-up tools from OSX86 Tools also, but still cannot get OS X to recognize my hard drive or DVD drives. Don't think the DSDT.AML file I'm using is the problem -- it's blkhockeypro19's file, but it's the one linked to by wmarsh in his Dell e520 guide, the one written specifically for RAID configuration. (It's apparently the same file used in this guide -- or, if not, it's written by the same person. Anyway, I downloaded it from wmarsh's guide link and replaced the one that came with the files linked to in this guide.)
Nevertheless, I'll check out DSDTSE and the links that blkhockeypro19 posted here for making a DSDT and try making one of my own just to be sure.
If anyone can think of another reason why my drives aren't showing up -- and also, why I can't shut down or restart properly (I now realize that shutting down hangs every time and I'm forced to power off the machine) -- feel free to chime in.