Dick, sometimes a pump that moves more water isn't always the best solution. Many skimmers were designed for a certain amount of water flow and adding more water may have the reverse effect of what you're looking for.
Also, if you're replacing a venturi pump with a non-venturi, you may again lose performance.
premiumaquatics.com sells a maxi-jet turbo venturi kit for about $5 that adds a venturi, and the maxi-jet 900 will just about match the output of your rio 600. IMO, a safer way to go. I'd almost recommend just getting another rio, but I just can't bring myself to ever recommend a rio.
Travis