I would not steer away from a 6Gbps SSD just because your mac pro does not support those maximum rates. You seldom see those max rates even on a computer that supports them. New SSDs are better in general, and more stable, than old 3Gbps-drives.
Just buy an OCZ vector or Samsung 840 pro of your favourite size and you'll get an SSD that will outlive your computer
If you're going to simply install an SSD into one of the 4 internal bays, I wouldn't go much higher than a regular 840 from Samsung. Even that drive will bottleneck on a SATA-II controller.
PCIe-based SSDs are really nice (and very fast), but they're really expensive and you give up an entire slot for one storage volume. Even a standard SSD would be a vast improvement over a mechanical boot drive.
The most important point on a SSD is fast random access, which ends up in high io rates. HDDs ends up with 200-300, but SSD can achieve thousands IOs. But the throughput is not high, most times below 100 MB/s. Only sequential throughput is limited by your controller, it is not needed to use a old SSD.
The Samsung SSDs are really good , the ones from Intel too.
Honestly, any that you can find, since the 2008 MP is SATAII.
Or you can get the Velo Solo X2 and put on any recent SSD and run at full speed. It also has a second SATA port for a second SSD (I have 2x512GB Crucial M4's running at highest speeds). These are used for red epic footage.
I have an 2x 80GB Intel X25 (RAID 0), which is SATAII for the OS and it's superb. I used the ODD ports.
The 4 drive bays are regular drives which are used for backup purposes and other stuff. So I have a total of 8 drives in there, 4 SSD's and 4 HDD's.
I'd be heartbroken to let this system go when the time comes.
You're right the Mac Pro 2008 only has SATA 2, so you could only get speeds up to 3Gbp/s.
We've seen great results from either the OWC PCIe SSD cards or just putting in a standard 2.5" SSD drive with a 3.5" Bracket. We recommend the SATA 2 Mercury Electra Pro by OWC.
In the UK we sell upgrade parts for the Mac Pro here.