Well you need to tell us the exact config...Intel or AMD
2gb dual channel means what?? 4 sticks of 512mb or 2 sicks of 1gb??? makes a difference has some if not most AMD64 board will not run those at 200fsb or pc3200...It will revert to pc2700...
4gb single channel will likely be 4 1gb module, again same as above...
now on average dual channel is about 5-10% faster on P4 rigs that need sthe bandiwdth and only 2-5% on AMD64 rigs that are not as bandwidth hungry....2gb versus 4gb likely wont make a difference unless you are a server or something...1gb is enough for most desktop uses...