Optiplex 755 has core 2 and that's getting near 10 years old. That's not going to be playing AAA games. Upgrading a prebuilt is usually cheaper. You can find a whole pc with i5 2400 for $130 or so. Buy a used 750ti and you can keep that near $200 without a psu upgrade.
Depending where you live you can find better deals locally at used pc stores, thrift shops or craigslist.
Hw/sl with a 960? I'm pretty sure if they were asking if a $50 optiplex 755 can be used as a base, they aren't looking at something that is more than 10x more expensive.