I've seen that there are PCI-e - SATA3 adapters, but I've also seen that this will most likely drop the SATA multiplier on my GPU from 16x to 8x and as I use my machine for gaming (I'm getting an SSD to improve performance of texture streaming in BF3, among other things) this isn't ideal.
I have also read (from an unverified source) that if you have a motherboard with PCI Express slots rated lower than 2.0 then SATA 3 drives wont operate any better than SATA 2 drives anyway and as far as I can tell my motherboard only has one 2.0, which is currently in use by my GPU.
So my main question is; how much difference will I notice between using the SSD at SATA2 and using an adaptor to run it at SATA3, and would you think it would be worth it overall?
you may notice some difference, however it wont be nearly as drastic as the difference you will notice going from a mechanical HD to a SSD. in game performance should not be affected by sata 3gbs vs 6gbs but load times in games and boot times most likely will be. its up to you if that trade off is worth potentially losing a few fps (many will argue that a pcie 2.0 8x connection is plenty for pretty much every single gpu card out there and they would have a point)
You can run the ssd in your sata 2 port but will run at sata 2 speeds as sata 3 is backwards compatible. But if you wish to use a pcie to sata adapter, your mobo is x16/x4 so your gpu will not slow down. Pcie 2.0 is 1GB/s per lane.
That ssd is rated at 550/510MB/s read/write while sata 2's limit is around 280/260MB/s. It is important to note that these are max sequential speeds and most of the time will hardly reach those speeds. http://www.legitreviews.com/article/1723/6/ (averaging 14-200MB/s) So will you notice a difference? No not really except transferring large files. As geofelt said the main benefit is access times, just plug it in your sata ports and don't bother with a pcie adapter.