Actually, the GTX750Ti is faster than the R7 360:
http://www.tomshardware.co.uk/gpu-hierarchy,review-33383.html but a good deal slower than the GTX950 (it's not in that chart but sits one or one-and-a-half tiers below the '960).
At such a low res you'll be OK with the '750Ti though, just get the 2Gb version, even at that res a 1Gb card is going to struggle with memory limitations in current AAA games.
The venerably PII 955 is still a useful gaming part, but you may just try overclocking the current one and see if you can get close to the 955s' speeds, just be VERY aware that you WILL need a better cooler to tame the heat!
If overclocking isn't your thing, you need to check the motherboard makers website to make sure it'll take a PII 955, even if it will, expect to update the BIOS before the upgrade.
Sourcing such an old part will mean a sweep through E-bay, Craiglist, or your local equivalent for a used component, it's VERY unlikely you'll come across any unused parts; Usual rules apply here, check the sellers feedback, don't bid too much, pay by credit card and there is plenty of well established sellers that trade these parts for a living, they're your best option for a smooth purchase and good service (provided their feedback is good
).