Note: This computer is not by any means good for modern gaming. I would not recommend buying an pre-built of this low caliber to upgrade it. There is no future pathing as OEM Motherboards have limited CPU support and it is on the old LGA 775 chipset.
Back to business! This beefy Compaq was never any good when my family used it as a Home PC. It came with Windows Vista; The OS you can use to delete the Recycling Bin. It was terribly slow and did not game well.
Just recently, I got into pouring a handful of money into it to breathe new life. I am giving it to a kind friend that has no money to afford a PC. This computer is good enough to run League of Legends and games of that caliber at Very High at 60FPS, no shadows. The resolution of the monitor I will give to her will be at 1600x900. The total amount of money going into it by me is at a low 35-ish USD. From what it what as, this is a huge boost. If any of you are looking to get a decent boost out of a old computer, then I might suggest you do the same.
Scower eBay. Do your homework on your computer; especially your motherboard. Find out what you can do for your pennies. It might do you some good.