Experts who know a little help me to see how these two computers exchange accessories to make a higher host and how to find second-hand graphics cards.
In fact, just add a graphics card to the first computer. If you want to play a large stand-alone game, it is recommended to use an N card, such as 9600GT 9800GT. If you want something cheaper, just 8600GT. Then, if you don't need to change the motherboard, it is recommended not to change it. There may be a mismatch.