The depreciation (private sale, about $300 tops) you're going to get out of that system (I am guessing $800 to a $1000 new) will far out weight putting a little more money into it to get it to do what you need it to do. Not only are you taking a $500+ hit in depreciation, then you've got to turn around and spend another $1200 on a pre-built and hope it will do what you want, or spend $900 on building your own system to get the best bang for your buck. That means spending nearly $2000 just to have one system do what you need it to do.
Your limiting factor is the GPU, the Geforce 230, and getting a $200 to $250 card will go a long way toward making that system perform like you're wanting it to.
Edit: One of these should do quite nicely: http://www.newegg.com/Product/Produc...82E16814133397
However the 300 watt power supply may not keep up with the additional power draw of the 560 (power difference seen in the link below), so get one of these to go with: http://www.newegg.com/Product/Produc...82E16817154026
GT 230 vs GTX 560: http://www.hwcompare.com/11025/gefor...force-gtx-560/
The 560 is about 5X more powerful/"faster" than the 230.