Anders, in your original question, you asked for a % difference. You need not look any further than the SR performance database, and make a comparison of the two drives (as seen here). The % difference in single user performance can be easily calculated. In the SR Office Benchmark, the Atlas 15K II wins by 19%........in the SR High-End Benchmark, the Atlas 15K II wins by 10%.......in the Boot-up Benchmark, the Atlas 15K II wins by 12%.......in the Gaming Benchmark, the Atlas 15K II wins by 18%.........the Atlas has a 15% sustained transfer rate advantage, and a 32% seek time advantage. If you ask me, this is a SIGNIFICANT difference in desktop performance, with the Atlas obviously being the faster drive. I do in fact think you would be able to tell a difference, especially with the feel and snappiness of the system. As continuum stated, this is mainly due to the seek times. I previously had a 74GB Raptor at one time, then had a 15k.3, but now have an Atlas 15K (1st generation). Many people here will disagree with me, but there is no way you can convince me that the 74GB Raptor is as fast as the original Atlas 15K. People might say "well, the SR benchmarks tell a different story when you compare the Raptor to the Atlas 15K" (as seen here). However, in my experience with the two drives, the Atlas 15K is definitely a faster drive. After using an Atlas 15K, there is NO WAY I would go back to the Raptor. My system feels much quicker and snappier with the Atlas. It goes without saying that the Atlas 15K II would be even better. It really depends on how much you are willing to spend for the extra performance.