First of all, which 1.67 do you have? The prior one with DDR PC3200 RAM, or the latest DL Superdrive, HD screen one with DDR2-533 RAM? The RAM is not interchangeable between these two models.
Second, before deciding where to buy, check the reputation of the vendor at resellerratings.com
MR readers in the USA most often recommend Crucial, OWC (MacSaler.com) and
Data Memory Systems.