I haven't been able to find enough details to figure out what video cards would fit in your case.
My advice is to find a forum with lots of other Asus Essentio owners and see what other Asus CM5570 owners are doing to upgrade their systems.
It's a lot easier to follow someone who's already upgraded and had success.
Well let me ask you this: Initially I thought the poor performance was due to my graphics card but after looking up the nvidia geforce 8600 GT the minimum power is 400w. So if I upgrade my power supply to at least 400W do you think that graphics card is sufficient to play the games I mentioned?