Hi there. I've been looking into clouds recently, maybe some of the research I've done could guide you towards a solution.
Perhaps the simplest solution is going from 2D to [url="http://stackoverflow.com/questions/6091468/how-is-a-3d-perlin-noise-function-used-to-generate-terrain"]3D Perlin Noise[/url].
I would suggest using billboard sprites as dpadam450 said. I'd recommend reading two papers, [url="http://nis-ei.eng.hokudai.ac.jp/~doba/papers/sig00_cloud.pdf"]"A Simple, Efficient Method for Realistic Animation of Clouds" by Yoshinori Dobashi[/url] (PDF), as he describes a way to represent cloud volumes. The second paper is on the actual billboard sprites, [url="http://www.markmark.net/PDFs/RTClouds_HarrisEG2001.pdf"]"Real-Time Cloud Rendering" by Mark Harris[/url] (PDF).
If you want really nice clouds, try [url="http://wiki.patapom.com/index.php/Clouds"]http://wiki.patapom.com/index.php/Clouds[/url]. It the most extensive documentation on clouds I've found to date, but it is very complex. Good luck.