hey. I was wondering is it possible to upgrade the 4th Generation Intel® Core™ i3 Processor from a alienware x51 and upgrade it to a 4th Generation Intel® Core™ i5 Processor?
Yes, most of the Core series CPU's from the same generation fit the same socket - LGA 1150 for 4th gen. As long as the first numeral in the product name is the same, the socket will be the same too.
For example: Core i5-4
670k , Core i3-4
330 , Core i7-4
770k are all compatible until you get into the 2011 sockets which are $500 + CPU's like the i7-4960X etc.