Dell Dimension E521 and 800MHz RAM?

(First, apologies if this question's been answered elsewhere - I've seen several references to it but can't see a conclusive answer after much searching.)

Can I replace the two 512MB 533-MHz with two 1GB 800-MHz RAM cards?

It's a Dell Dimension E521 with AMD Athlon 64 X2 dual-core processor. Both Dell and Kingston sites indicate 800-MHz is compatible but Crucial indicates its not and 667-MHz is the maximum.

The service manual says 533-MHz, 667-MHz, 800-MHz (when available) are all supported. I'm not sure what 'when available' means here. Should I replace with 667-MHz or 800 M-Hz memory?