I would go for Western Digital drives, since you get a very good quality product (the best actually) at a good price with excellent support and a very good waranty.
here is prob what you are looking at:
western digital 400ab 40gb , ( 2mb ) - ultra ata100 - 5400 rpm
western digital 400bb 40gb , 8.9ms , ( 2mb ) - ata100 - 7200 rpm
western digital 600ab 60gb , ( 2mb ) - ata100 - 5400 rpm
western digital 600bb 60gb , ( 2mb ) - ata100 - 7200 rpm
western digital 800bb 80gb , ( 2mb ) - ata100 - 7200 rpm
Again I'll go for the 7200 rpm drives since they show quite some speed increase. Hope this helps...