Hi. Before I upgraded my head unit, I upgraded to the Polk dB651 speakers. Distinct improvement over the stock speakers, even with the Radio 10.
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So far as upgrading the OEM speakers is concerned, it doesn't take a lot to improve over them. The Polk dB651s are a forum favorite.
I have a slightly used set for sale, since I have upgraded again.

But, they're inexpensive, and I doubt I'll unload my set. I'd send them to you for shipping plus $2.
Anyone interested, PM me. Would be nice if you have a PayPal account. This is a freebee, basically.
While I'm at it, I've got a Radio 10 I don't need.
UPDATE 7/10/2009 --- Both the speakers and the radio have apparent takers.