I need help finding a power supply?

i am building my first computer i have most of my parts i just need a power supply and i dont know how big of a power supply to get so i was looking at a 600 watt or a 750 watt i dont know if that would be too much or not enough so here are the parts that i have Cooler Master HAF X ATX Full Tower Black Case, XFX nForce 680i LT SLI Socket 775 Motherboard, Intel Quad Core Kentsfield 2.4Ghz Q6600 CPU, OCZ Vista Upgrade Edition 4096MB PC6400 DDR2 800MH, Thermaltake Frio Dual 120mm Universal CPU Cooler, evga geforce gts 450, Western Digital WD1500HLFS VelociRaptor Hard Drive so any info will be great thank you and have a good day