garmin forerunner is the beast you are after ... Sunnto also make some but theirs involves lots of bolt on's
check out garmins website they have a little wizard that helps you select the right device
That the one i use (Garmin Forerunner 301)
tells you how far you've run, calories etc and you can plug it into your pc and it provides you will a detailed analysis of your run (split times etc) it also works with google earth so it provides an ariel view of the route you have run.
got mine off ebay about 18 months ago for £100
I went for a gait analysis a few years ago when traning for the London Marathon at sutton runner i think, i used to have trouble with "shin splints" a common ailment for runners. I was recommended Brookes Addiction and have stuck with these ever since.