curiosity, I prepared for my GREs with the Kaplan book and it helped me a lot. I'd say self-preparation is definitely enough, but again, it depends on the time spent on preparation, your existing vocab and quantitative skills and what not. Here is one useful website that I used http://gre-download.blogspot.com/2009/12/barrons-gre-high-frequency-333-words.html. If you have a smartphone, download free GRE apps so that you can learn words on the go.