With what you have now you won’t get anything decent at gaming for less than $800.
You’re looking at a power supply, motherboard, processor, RAM, and video card at the absolute minimum. It looks like you’ve got a half-tower case so you’ll probably need a new case as well to fit the newer video cards.
If your hard drive is over 2 years old you’ll want to replace it with a newer SATAII just for the speed, reliability and space needed for modern games.
You might get away with it at $600 but it would be using older generation parts and when you wanted to upgrade again in a year or two you’d have to rebuild the whole thing again.