Your motherboard has SATA II connectors for SATA II hard drives. This means that while you can purchase any type of SSD you want (either SATA II or SATA III), you will only experience SATA II speeds on your motherboard.
If you choose to purchase something like an Intel 320, then you won't have to worry about it, since the Intel 320 is only SATA II
But if you want something like the Intel 510 250GB SSD, which is rated for SATA III, then you'll only experience 50% of the speed that the SSD would normally offer in a newer and faster board.
In the same way your board limits you with your USB endeavor, it is also the limiting factor with new SSD technology. I would say that limitation is much less, given that SATA II speeds will be much faster than anything else on your board.
I would recommend upgrading your board and CPU as soon as possible if you're interested in some of these new technologies. If you're on a budget, consider a Core i3 processor and a small MicroATX board.
A SATA-III SSD running on a SATA-II interface will still greatly outperform any RAID-0 configuration as far as reads/writes/latency go. A SATA-III SSD running on a SATA-III interface may violate causality
You could use an ssd as your main and only drive if you wanted to.
After installing your ssd inside your case start your pc. Immediately go into the System BIOS and change from IDE to AHCI mode. Also check the BIOS and make sure the ssd is listed as the main/primary/boot drive. Save the configuration and continue with the bootup. Install Microsoft Windows 7. Install drivers. Once you are satisfied that everything is working properly continue with the other installations and anything else that needs to be done.
1) on Sata III SSD plugged into a SAT II port.
.. Poster indicating 50% difference, another poster "Great improvement" Bunk
You MAYl see a 100% gain in Sequencial performance ONLY. Sequencial performance is the LEAST important matrix for a OS + Program drive. IT is the Random small file (4k) that is important which will rarely saturate a sata II interface. Sequencial's ONLY become important when the SSD is used as a "work" drive and working with LARGE file structures. When looking at performance, use PCMark vantage benchmark which is much closer to real life.
This is a dated (old) link, but shows How Sata III, on a Sata III interface compares with the same SSD on a Sata II inferface - Not a real biggy. http://www.anandtech.com/show/4421/the-2011-midrange-ss...
Some off the newer SSDs will show a bigger differece, but still not any where nere a 50->100 gain.
Yes you should have trim. Windows 7 will enable trim as long as the SSD uses AHCI (msahci driver). For Intel systems the latest ahci driver (iaSTor) can then be installed, which improves performance over msahci (Intel's driver should be ver 10.5 or later).
Also for Intel systems you can use raid, as long as the SSD is NOT a member drive of a raid0/1 configuration and trim will still be enabled and passed to the SSD.