PC2100 vs Pc3200

I currently have a 256mb pc2100 memory module installed. i am looking to adding a 1gb module in and i was wandering which of the 3 configurations people would recommend?

1) adding a 1gb pc2100 module in and having about1.2gb pc2100 memory in?

2) replacing the current module with a 1gb pc3200 module which the motherboard will cater for?

3) replacing the current module with 2 512mb modules pc3200 to take advtange of dual channel memory?

Any help is greatly appreciated

Thanks

Sam

one of those two was the one i was thinking of but wasnt sure how much extra memory you would need to be equivalent if at pc2100

thanks