Currently I run my Crucial SSD on a sata II port and get about 300 mb/s read/write. Is it worth getting a sata III 6gb adapter to see if I can get the full 500 mb/s on my system and will I notice any difference? Have seen a PCIe x1 SATA III adapter on Novatech for £20 but will I also be able to boot from it on my MSI P35 Neo F motherboard? I think I have 2 x PCIe x1 slots but one is taken up by graphics card heat sink.
I did find a good SATA II vs. SATA III benchmarking article and I was going to post a link to it, but now I've forgotten where it is. I'll see if I can find it again for you.
Have you made sure that your system is disk I/O bound? If it isn't, then providing a faster interface for your SSD is unlikely to provide any overall improvement, although some operations may be slightly quicker. You should really do some system monitoring to see if your SSD is currently causing a bottleneck. If it isn't, you'll likely just be throwing money away buying a SATA III card.
Have you done any system monitoring? What is the average disk utilisation? When I did performance tuning on mainframes and minicomputers, any disk utilisation of 30% or more was considered "high" and in need of rectification. I normally liked to see disk utilisations of no more than 15%. However, things may be different in the PC world.
Other than benchmarking no, I have had the odd peek at task manager performance tab but that usually only gives current utilisation over a minute duration. Utilisation is no more than 3% on that graph.