Performance wise, at DDR400 how much of a difference do timings make?

I'm getting 1GB of memory, so if it's running stock at DDR400 and one stick has say 2-2-2 (expensive, great timings), while another has maybe 2.5-3-3-7 (cheap, alright timings?), how much of a difference would there be in performance?

If the platform type matters, it would be with an Athlon 64.

For clarification purposes, S939 or S754?

754, 3200+, K8V SE Deluxe