IMO...with the exception of replacing with a SSD...there is no particular speed gain for Windows by replacing hard drives.
I replaced all my PATA/EIDE drives with SATA drives...simply because PATA drives and connectors were going away and the prices for SATA drives are simply cheaper now than those for PATA drives (items not carried in vendor inventories routinely result in higher pricing).
I replaced my SATA boot drives drives with SSDs...simply because I was curious and I don't believe in ignoring the possibility that Windows may perform faster/better on the newer drives...and I feel that SSDs will eventually replace PATA/SATA hard drives as the prices come down and the capacities increase.
That's how it was with SATA/PATA and I expect the same for SSD/hard drives.
IMO...the quick fix for a slow system ..is to maximize the RAM installed...but this won't overcome issues that have nothing to do with a system perfoming up to its expected capabilities. It won't overcome limitations of the motherboard/CPU.
Simply put...newer components tend to outperform their older counterparts and nothing will overcome that.
But...take a look at Slow Computer-browser Check Here First; It May Not Be Malware - http://www.bleepingcomputer.com/forums/topic87058.html .