While I usually think just about anything is worth testing to see how it works for you, I'd hate to see you throw $2k into this and not get the results you were hoping for.
For starters, I could see this working better if you had a sticker INSIDE the taxi where people would at least see your ad long enough to be able to write down your number if they needed your services. I drive by all kinds of buses and taxis with ads on them each day and I couldn't remember one of them 5 minutes later if you paid me! ;-)
Personally, if I were in your shoes, I would do two things...
1) Approach a local newspaper of any size and see if they'd be interested in running a weekly column with tech tips and advice for their readers. If you can write a halfway decent article that's of benefit to their readers, free advertising like this with your contact info at the end of each article could be priceless.
2) Find other local businesses that deal with business computer users (like an office supply store for example) and see if they'll place a flyer of yours in every delivery they make. Pay for the printing and tell them you'll pay them a finders fee of $XX for every new client you get.
3) I'd place lots of signage on my vehicle so it'd be noticed where ever I go, and if you have some down time - park in a high traffic area where others are likely to see it - like a fast food place on the main highway or the entrance to a local shopping center.
Oh, and I'd really consider having an email newsletter prospects can sign up for (one where you regularly give out good, free advice) because the problem with computers is no one ever knows when they're going to have trouble - so you need to be the first one they think of when they have a problem - and communicating with prospects through a newsletter is a great way to do this.
Hope this helps some!
P.S. Having just read a post about business card displays here, why not see if you could leave a holder with business cards in it in the back of each cab? Google "vehicle business card holders" and you'll find things like http://www.vehiclecardpockets.com/
that should work for you here.
(Just noticed at the link I just posted there's a testimonial from a computer tech using their product)