I can vouch for the DiamondMax 10 (7,200 RPM,SATA/SATA-II, 16MB buffer, 300GB capacity). It's a good purchase due to the SATA-II support, meaning not only do you get SATA for today, but with the appropriate controller you get Native Command Queuing (NCQ) for more efficient transfers.
Note that the DiamondMax 10 250/300GB SATA drives with 16MB buffers are essentailly the same drives as the Maxtor Maxline III drives of the same specs. I believe this goes as far as simply being different labels for two markets and two price points. AFAIK DiamondMax 10/Maxline III are the only drives on the market that support SATA-II at the moment. MOre are sure to come as this interface is looking to replace the initial SATA implementation.