I just purchased a new ASUS P5AD2-E Premium motherboard and I'm about to connect my hardrives.

I have a new Maxtor Diamondmax 10 160GB SATA 7200 RPM drive and a Quantum U160 SCSI 10k RPM drive.

Which should I go with? I'm assuming the SCSI will be faster, but will it be much faster? Since, using SCSI will take some work off the CPU...is it worth it? will I notice much performance difference?

Hi
I would use the sata drive for apps and storage and the scsi for os.
If I didn't have to add a sata adaptor to get sata running, that is.
The scsi will have far less latency than the sata drive, so it should give a snappy response to user input when used as the os drive.

The seek time on a 10K drive is roughly 30% less than on a 7200 RPM drive. You should definently choose SCSI for the OS and swap.

Is the percentage still that high for drives such as the Maxtor Diamondmax 10 300gb with NCQ?

I doubt it. I think that Maxtor will have a bunch more cache than the Quantum. Check the manufacturer spec sheets for the drives and compare. They may be not entirely accurate, but they probably both lie the same amount.

The on-drive cache amount is a useless number. I just posted a paper on another thread that shows that there is absolutely no difference in real life when it is >512KB.

The disk buffers of your OS use all the RAM you have free. So it can be anything from 10MB to 1GB. The measly little cache on your HD has very little effect on performance unless you have some very specific needs or you have used up all your real RAM.