Here is one example of some of the high performance fans available..

http://www.ackits.com/index.cfm?fuseaction=Electric

Since the first most limiting factor in the physics of an AC system is the air flow across the condensor... and with you losing efficiency due to changing to 134a.... the most fan you can mount/afford may be important to your comfort level... and to the longivity of your compressor ( due to its effect on the amount of time the compressor has to run at any given inside car temp ).