No you can't run hi res cards on the old computer. The 240w psu cannot handle cards like that. You have a monitor that is worth more than your computer! Best bet is to buy a new computer to make use of the great monitor. After all the computer is a 5 yr old business model that is slow. Put it on your holiday list!
Forgot to mention look at home computers with discrete video cards like the XPS not Optiplex business models