Just as a side-note before you embark on a journey that won't net you much improvement in anything:
Deleting old preference files and cache files will not gain you any worthwhile hard drive space back. Even if you had 100,000 applications' worth of old preference files, that may get you back 300 megabytes or so of space -- in other words, you wouldn't even notice. For a more reasonable amount of applications, I'd bet you'd see about 40MB of hard drive space freed. In 1994, that was a big thing to do a dance about. Nowadays, it's nothing to bat an eye at.
Deleting old preferences files and cache files will not gain you any worthwhile increase in speed, either. Those files are sitting there, idle, taking up minimal amounts of room and doing nothing to impact performance of your computer. Your computer will not perform a single percentage point better than it does now if you delete those files.
The one and only reason to go deleting old preference files is to keep a "tidy" hard drive (only in the mental sense, since the computer has no notion of a "tidy" nor "untidy" hard drive) -- in other words, peace of mind. I'm not discrediting this reason at all, nor am I saying it's any more or less important than anything else -- I'm simply saying that embarking on this endeavor will be for human reasons only... for peace of mind... for "tidiness"... for a cleaner mental "picture" of your hard drive.
Just my $0.02.