I think you should definitely get some fans; a front intake and a rear exhaust would be good.
Second, that Thermaltake "450W" PSU may be near junk if it's an older one with a little voltage switch on it. A quality modern PSU has some level of 80+ certification; it's not so much for the efficiency, but that it was tested at 100% of its label, and not only didn't die, but was also efficient (meaning it probably has some reserve capacity as well). Of course, 80Plus tests at an unrealistic 23C, but I still think this is a passing rule of thumb, because cheap junk would die even under those conditions. Better PSU brands include Seasonic, Antec, Corsair, XFX, FSP, and Enermax/LEPA. Not every model from those brands is great, but none of them outright suck. A 380W or 430W Antec Earthwatts, or 400W or 430W Corsair Builder would be sufficient for either of those cards.