You need a quality 450w power supply for a GTS 450. 750 watts is way overpowered even if you plan on running 2 of those cards. For 2 cards go with 600 watts. Corsair, Seasonic, PC Power and Cooling, XFX, Silverstone, Enermax, OCZ and Antec are all brands you can count on for quality.
aside: after researching hyper-threading performance, i disabled HT in the BIOS for my i7-920. Some things work better with HT, some work worse. I care more about low-latency for not too heavily threaded things. (The win7 dispatcher got better about 'parking' SMT (hyperthreaded) threads, so this research may be outdated.)