2 pairs of 512MB (2x256) dual channel ram or 1 pair of 1GB (2x512) dual channel ram.?Assume that all the ram will come from the same vendor and have the same latencies.
more ram = better chance of NOT using page file. If both sets of ram have the same specs, I see no reason why you wouldnt want to get the 1gb over the 512mb...that is unless you dont have the moola.