Upload & Sell: On
Here's Nikon's "approved card" list. Hasn't been updated since 2009:
Does list cards up to 300X, and has a footnote that says "All cards can be used, regardless of speed."
Also, a quick note on their support site about card speed, which pretty much reflects what is said here, and more importantly doesn't make any warning about impact to the battery...
On the Lexar site, they make specific mention that their latest 1000X cards are compatible with the earlier CF standards: "Compatible with UDMA CF and CF devices" (and no warning about impact to the battery).
I wrote a blog post about the card performance not too long ago (actually two, since I screwed up the first test )
Lastly, refer to Rob Galbraith's excellent article about the Lexar 1000X card, which implies that the UDMA support of the D300s (and I'd guess the D300 as well - it does support UDMA as well) are capable of handling the faster cards.
I think you misunderstood him/her, or they misunderstood you.
Net: I wouldn't worry about it. Buy the fastest card that meets your budget and desire for quick pulls from the card. If you're a maxing the FPS and hitting the camera buffer (usually sports or birds-in-flight), then you might get a slight benefit since these cards will clear the buffer faster.
(apologies for the Moby Dick post - the coffee was strong this morning...)