The best value now is the SFF dell or HP-Compaq. Most are sandybridge i5-2400 3.1ghz or i5-2500's. It's mostly that they include windows 7 64 or 10. These are $160cnd ($120us) Locally. Your best bet is to look for a local store or someone who is selling them locally as the shipping online for a bulky item like that is costly. It's used so you also want to see it working first. It's smart to ask to see it with the case cover off so you can confirm it has a standard atx plug on the board, and a pcie-16 slot.
-There is also a HP-Compaq micro-atx tower. It fits a standard atx power supply and also i know that a HD-4870 fits so most cards like a GTX1070 will fit as long as you don't get the OC triple fan version that is longer. They are usually $80-$120 more than the dell sff.
-You must transplant the system into a bigger case with a new power supply. A new 2nd hdd is good too as most only have a 160gig in them.
Cloning the 160gb to a new SSD would also be good.
If your going with the GTX1080 or RX480 crossfire, of course the older sandybridge wont work well, and now it's a much more expensive skylake K cpu and so on. The i7-6700K with a stock 4.0ghz clock is likely the best value.
-DO YOUR RESEARCH, THERE IS A ULTRA SFF THAT USES A LAPTOP BOARD AND POWER BRICK, OF COURSE THOSE WONT WORK. BE CAREFUL OF WHICH COMPUTER YOU PICK OUT.