XP can handle 4gbs of ram, so if you turn the pagefile (virtual memory) down to 0 you can theoretically have 4gb of physical ram - it's not reccomended though.
As to the dual channel question, most mobos only have one set of dual channel slots, but some modern ones have two (usually 1,3 and 2,4 but it varies) and that must be what you're running. My mistake.
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