A slight buckle could probably be straightened out by a competent bike shop mechanic, so there may be no need to replace it. Said mechanic will advise you. Try to find a small local bike shop (LBS) with reccommendations, rather than a chain. Ask for reccommendations on here and you'll be swamped with them: just tell us where you are.
Next problem: a six speed wheel will have a screw-on freewheel, and this gives us an idea of the quality of the bike you have. Unless it's more than ten years old, it's probably at the low end of the quality scale. So if you want to go faster on the road, you're probably better off buying a nicer bike altogether, rather than putting silk purse wheels on a sow's ear bike.
That's because more modern bikes of all but the lowest quality now have seven, eight, nine or ten-speed cassette freehubs, rather than screw-on freewheels. The wheels have completely different hubs, so the two systems are mutually incompatible. You could fit one of these cassette wheels with an appropriate cassette, but you may then have difficulties getting the gears to work nicely, depending on what sort of shifters you have.
Finally, if you just want to go faster without spending too much money, your best bet may be to get that rear wheel straightened (or replaced if necessary, perhaps with another cheapie screw-on freewheel hubbed wheel) and fit narrower slick tyres, and pump them up good and hard. That'll make more difference than you think to your speed.
Front wheels, by the way, are pretty much interchangeable as long as you get the right size.
