Computer RAM Question

[Unoriginal Name]

Currently I have two of more four RAM slots occupied with 2x1GB at 5-5-5-12. If I was to add RAM to the other two slots would I need to stay with 5-5-5-12? How much can I vary from 5-5-5-12 without hurting computer performance? I was looking at some RAM at 5-4-4-15. Would this work or no?

Thanks.