I'd take some caution in buying the cheapest domain you can find. I bought one at killer prices at Registerfly.com and had horrible service, and they eventually "stole" my domain and put an ad-farm in it's place without notifying me. After threatening them with lawsuits for copyright infringement, they finally gave my domain back for the rest of my paid few years, and said they had made a "mistake."
When the difference between reputable companies and cheap ones are a matter of just dollars, I'd go with ones you have heard of, like GoDaddy. I have also bought a ton of domains through Yahoo actually, they have given me fantastic service, they charge around $10 a year.