I was considering buying Sony Vegas 9 Pro, and I built a PC with an AMD Phenom II X4 965 BE 3.4GHz Quad-Core processor, 4GB DDR3 RAM, Radeon HD 5770 GPU, 1.5TB HDD with Windows 7 64-bit. I was wondering what I can do to make my computer edit HD videos smoothly. I never edited HD video before, so I don't know if my PC can manage. Thanks!
It will come down to the codec used on your source footage, but you should be fine with what you have.
Core i5 and Core i7 would have been a better choice if you're doing a lot or heavy-duty editing but unless you're working with Canon 5D footage (uses H.264 codec to capture so it needs some CPU grunt to decode) you're good to go.
If you're using Sony XDCAM-EX footage (very high data rate footage) you'd ideally need to run the files from a RAID 0 to keep the data rates up.
Also I'd look into putting in another 4GB RAM when you can afford it.