There are some options here. Your i5-2400 pulls 95 watts. You can buy an i5-2500T for around $75. That CPU only pulls 45 watts. Here is where the problem is. According to Calculatorpowersupply.net That rig with a single 7200rpm HD, 4 sticks of RAM, 4 used USD slots, the i5-2500T, and LP GTX 750ti should only pull 199 watts at 90% full load. You can also go with a SSD to drop that power even more.
If a CPU upgrade is not something you want to do. The best video card would be a AMD R7 240. It's OK but not good enough for games like the new patched WOW and up.