The GTX 550 Ti and HD 7750 do not offer much improvement in performance over the HD 5750 to justify spending money on it. See link below for some benchmarks. The HD 7750 only offers a substantial increase in performance for Crysis 2 and the GTX 550 Ti only offers a substantial increase in performance for Star Craft 2. Note that the test rig has an Intel Core i7 920 @ 3.8 GHz so your FPS will be lower.
For gaming you have more than enough CPU almost no game uses more than 2 cores and there's no need for a i7 . I have a HD 6850 with a Phenom ii x2 555 unlocked to x4 and can play MW2 MW3 BF3 SC2 all high settings on 1080p at 60fps and the good thing is that the 6850 runs with a cheap 500w psu or a semi-good 400w psu no need for 800w 80plus Saludos desde Argentina.