Do you have a budget to work with?
From the sounds of it your doing a lot of upgrading or a new build. With the limited info you have given another 2gb of ram would make a huge difference. For a 19inc monitor you don't need sli or crossfire.
As for operating system I would say windows 7 64 bit would make it more future proof its a lot faster than vista and runs better.
If your willing to spend a fair amount of cash for keyboard or mouse check out Razor products. I have a Lycosa keyboard and Lachesis mouse from them and all i can say is its like sex in yer hands. http://www.razerzone.com/
Sadly i cant advise on the gpu as don't know much about them.