So this one is a bit strange. I just upgraded my Radeon 4850 512mb for a 6950 2GB. Nothing else changed. I ran the Windows Experience Index before and after the upgrade.
The first thing I noticed when I rebooted was that it took forever to boot, much longer than normal. Steam also loaded MUCH slower than usual.
I then re-ran Windows Experience and found that my hard drive rating went from 6.8 to 5.9??? Graphics went from 7.3 to 7.8, no other numbers changed.
I have a 40GB Intel SSD as my primary drive and a Samsung 1TB drive as storage.
I did NOT upgrade my power supply, currently have a Antec 380 Tru Power unit, and I realize that I was probably going to be underpowered, but I have a hard time believing it killed my hard drive performance like this. No crashes, games are running fine (note loading takes longer in all cases)