Want to build a new i7 based PC

which i7?

there is Socket 2011 and socket 1155
500W is barely enough for the 1155

2011 is more powerful than 1155 (about 50%)
2011 requires far more power and can be very finicky about ram
if doing a 3820 and 4 sticks ram and a few hard drives and NO over clocking a very good quality 80% or better (NEW) 500W would be just enough. if your power supply is left over from your old system and older than 6 months i would NOT use it

ivy is 1155 or slower than Sandy-e (2011) will be replaced in June by Haswell
in 4th quarter Sandy E will become Ivy -E

3770k/3570k is ivy/1155
3820, 3930k, 3670X is 2011

If you plan to over-clock the 3820, that cooler will not be enough to keep the machine both cool and quiet.