Most of what drags down a PC in gaming is video processing, graphics cards not only take some load off the CPU, they do high end vid processing in bad ass milti core GPUs with gobs of dedicated memory and they do it whole lot faster to max the screen image quality.  After all its all about image quality.  So it goes without saying that you need no more graphics processing then you have display.

Look for the newest GC models with latest Direct X, high clock speeds, high bit rates, high core count, a minimum 8 GB of GDDR5 memory and lots of cooling fans.  As they say, it ain't working if it ain't cooling.  Again be sure to match to your display.

In all but extreme gaming, a <$500 invesment in a high end GC for a spare PCI Express slot (or two) will do for most people, provided the PC is modern and capable on its own.  Best bang is to do a little home work, buy a new pro level PC, with the necessary open PCIe slots and expansion capabilities and your choise of CPU(s), then get your choice of GC.  Any spare cash is best spent on fast solid state internal drive(s) and more memory.

The law of diminishing ROI definitely applies to gaming PCs and graphics cards.  You can also be sure regardless of how much you spend, the system will soon be obsolete.