MacPro 2010 adding SSD drive I am looking to gain some performance enhancement by adding an SSD drive to my 2010 MacPro. I can mount it in a hard drive slot or get a PCI card and mount it there. 

The SATA speed of the Mac is 3Gb/s, but the PCI shows 'Link Speed:5.0 GT/s' which (I could be wrong) is 5G 'transfers' per second, since the encoding is a 10 to 8 ratio it's really 4Gb/s? 

It seems if I'm going to make the effort, it's worth it to gain a bit of speed, and not lose a HDD slot. I've googled quite a bit, and not seeing this discussed anywhere.
I have a 2008 Mac Pro and I swapped the main HDD with an SSD and re-installed OS X and I saw a HUGE performance increase. You'll always see a performance increase when going from HDD to SSD simply because it's a different class of hardware. SSDs are, as their name implies, solid-state. There are no moving parts and there's nothing that physically degrades over time. HDDs start to wear immediately; while it's a slow process it does happen for as long as the drive is in use (that's why most operating systems tend to turn HDDs off when not in use - it prolongs their life).

That's my experience and my recommendation (my recommendation is definitely do it).