So how much longer will this card be a decent card to play games on medium to high settings? How much longer should I keep this card before upgrading, and what would be a good upgrade?
A good GPU would be a GTX 970. They are rapidly decreasing in price, and you can find then on ebay for about 175, but I would bet that they will decrease a fair amount after the GTX 1080 and 1070 are in full swing.
The GTX 750 Ti is still a pretty decent card. For how long it will be able to play medium - high it is hard to say and will differ from game to game.
For upgrading, keep it until you feel that it isn't performing to how you want it to perform and then upgrade.
About to upgrade to an Asus P8Z68-V Pro and Aerocool 1000 case, 16GB Ram, 250GB SSD, 2x 500GB HDD (looking to add a 2TB drive soon) i5-3340, Corsair 500W PSU. Just wanna know what a good option would be to upgrade to that would give me a significant boost in performance over the 750 ti
A good GPU would be a GTX 970. They are rapidly decreasing in price, and you can find then on ebay for about 175, but I would bet that they will decrease a fair amount after the GTX 1080 and 1070 are in full swing.