All these sites are good but please be very carefully and read the licensees that come with them.
Not all on these sites are free to use for Commercial Use and many have rules on how you can use the stuff like full credit with the product in full view when releasing.
some are under attribution 3.0 and some are under attribution 2.0 both are diffrent.
Some are attribution noncommercial so please be carefully when using these sites.
Best s4real
System spec :-
Amd fx4100 runing at 4.0,8gb rams,3tb harddrive space, geforce 450gts