First things first - are you running W2K or XP? If not, the W9x family have real difficulties with more than 512MB ram, and can actually run slower.
The next issue is what type of ram your motherboard will take.
The simplest option is to ring Crucial, and take their advice. They have an excellent reputation, and will exchange any ram that doesn't work (if they recommended it).
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They will also explain what difference, if any, your various option will make.