[H] PCI Bus at 3MB/s

[Winterlight]

I have a older P4 3.4 rig I want to add some SATA ports to. I don't 
want RAID. I will probably get a new/or used 4 port Promise 
controller. Is there any good reason to pay for a 3MB/s controller as 
opposed to a 1.5? Seems to me a PCI bus couldn't handle that kind of 
bandwidth anyway..... am I right?