my current system is fine, but I am working with a large database, I want to buy an ssd and dedicate it for my database so my windows7 will boot from my original mechanical drives but I want my database to be on ssd is that do-able?
And would I get the speed I want for the database if I were to have windows on the mechanical drives and the database only on the mechanical?
There is not reason you couldn't set things up this way.
I would try to get a drive large enough to hold windows and the database both on the ssd if possible.
There may be some reasoning against storing a database on a consumer grade ssd especially if it is getting a lot of use. I would research that a bit before trying to go this route, and definitely keep a backup of it if you do end up choosing to do this.