1. What wattage would be adequate for the above-mentioned components? I initially budgeted for a 500W PSU
2. Do all PSU's come with 220V/115V selectivity capabilities? And if not, is it possible (and safe) to use some sort of 'voltage alternator/transformer'? I ask because I am ordering from an American supplier and my country uses the 220V/50Hz setup.
Well, there are a few things I'd change before we get to the PSU.
CPU: That's supposed to be the X4 965 (not the X2). Any way, what you should be getting is the X4 955. The 955 and 965 are the same chip and overclock almost identically. Save the $20+ and get the cheaper one.
Mobo: I highly recommend getting a USB 3/SATA III board. They're not too expensive, but they'll add a lot of future proofing. Two of the good ones are the Gigabyte GA-770TA-UD3 and GA-790XTA-UD4.
GPU: I would spring for a 5770. The 4850 is an older card and you should really want to be current
Sound: You really don't need a discrete sound card. Onboard sound is excellent now, and your not going to get much value out of a sound card. I recommend leaving it out and seeing if you can live with onboard.
PSU: 1.) 550W would be fine. Get any unit from Antec, Corsair, Silverstone or SeaSonic.
2.) I have no idea.
EDIT: We could probably offer a lot of good advice with the build if you let us know what the budget is and what the computer will be used for. Take a look at the guidelines (the link's in my signature) at all the information we typically want to have before giving advice.
The questions is solved with the answer from MadAdmiral.
Just wanna add, for future upgrade purposes, choose a PSU with more than 1 PCIe output. Even if you are going for single GPU. A HD5830 or higher needs 2.
Hmmm...some PSU have automatic voltage detector and switching to the correct voltage automatically, some have a manual switch, some don't have at all.
I think you won't get any problem if you get any PSU from the well known brands.
I have tried to read in my corsair manual book and it said it is compatible to both 110V 60Hz and 220V 50Hz. I can not find any switch so I assume my PSU has the automatic voltage recognition.
Honestly, I can't think of anything else to add with that budget (over than what I've said above). I would suggest you consider dropping the X4 955 to a cheaper CPU (like the X3 425) if that helps you afford the HD 5770, but that's really up to you.