the i5-3350p is still a very decent processor. why do you think you need a more powerful cpu? what programs are you running that you would like more speed in? if its for gaming, even an overclocked i7-4790k isn't going to give you an appreciable performance gain.
there are plenty of 3570k processors around, though they usually cost more than the 4690k if your buying from a retailer. you will want a z68 or z77 motherboard so you can overclock the k processors.