I'm afraid you won't get very good results no matter what you do, because the source materialVHS tapesare so poor-quality to begin with. I've never availed myself of these conversion services, but I suspect that the best of them can probably do a better job than I ever couldafter all, they're much more experienced, and they have tools I don'tso that's the route I would take.
If there is more than one such service in your area, have each of them convert one tapethe same tape in all cases so you have one basis for comparisonand see if there's a difference in quality between them. If so, take the rest of the tapes to the one that did the best job. Just keep in mind that they are not miracle workers, and the end result is going to look pretty bad compared with HDTV, Blu-ray, and even DVD.
Another option is to get a VHS-to-DVD dubbing deck, which is probably the easiest way to do it yourself. We haven't reviewed any such decks in quite a while, but Robert Silva maintains a list of currently available and recommended VHS/DVD dubbing units at About.com.
If you have an A/V question, please send it to firstname.lastname@example.org.