The only real difference between pcie 2 and pcie 3 is that 3 has double the bandwidth available. Considering that a gtx1050ti won't even come close to saturating the bandwidth of pcie 2 x8, nevermind the full x16, there's not an issue.
Nvidia cards are backwards compatible from pcie 3 to pcie 1.1, Amd cards are backwards compatible from pcie 3 to pcie 2.3, you'd need a very old amd k5 or lga770 or prior to see pcie 2.2 to 1.1. The first gen pcie 1.0 is standalone and basically proprietary, it takes pcie 1.0 cards from nvidia or AMD/ATI only.
Any newer motherboard (about the last 10 years or so) will be running pcie 2.3 or greater. So as long as you don't backslide and try using a very old ATI x1900 or similar, you are good.
There is one addendum to this, the bios. And this generally applies to 3rd party vendors such as Dell or Lenovo or HP. Because of use of hybrid uefi bios, newer cards can have issues with the legacy/uefi bios on those boards, this is especially true of the Maxwell gtx750ti. There isn't many reported issues with aftermarket motherboards as they use a full uefi or legacy bios, not a hybrid.
All that boils down to 1 thing. If a gtx750ti worked for you, anything will.
With that older i3 (3350T?) which are all truncated, chopped, lower voltage and performance models, you'll have a hard time justifying the expense of any gpu stronger than a gtx1050/Rx560 as the cpu will bottleneck considerably in any cpu depending game.