Just thought Id post some useful information from HobbyTalk that we were talking about back in October on this issue.
Copper and Silver have such a close resistance that a copper bar only has to have 5% more cross sectional area to match the resistance of a silver bar. What this boils down to is a Silver bar that has the dimensions of 0.25" x 0.0625" will have the same resistance as a copper bar that has dimensions of 0.25" x 0.065625", thats a very small thickness difference. So basically if a copper bar is noticably thicker than a silver bar that slightly thicker copper bar will have less resistance. Or if your bars have the same thickness the copper bar will only only have to be 0.2625" compared to a silver bar that is 0.25".
Here is a list of metals and their resistance