Now a days people looking for upgrading their computer are getting confused between Athlon and Pentium.

Generally, in my experience, you will get more bang for your buck from AMD processors. However, a price range is a good place to start. I would recommend the following, depending on how much you want to spend on CPU ONLY:

< $100: AMD Athlon XP (Barton core)
$100-175: AMD Athlon 64 (2800 or 3000)
$175-250: AMD Athlon 64 (3200)
$250-350: AMD Athlon 64 (3400)
$350+: AMD Athlon 64 (Socket 939 based)

Please realize that, on top of the CPU cost, you have the motherboard and all the other components. The more you spend on a CPU, generally the more you will end up spending on a MB and possibly RAM; the other parts are not as tied to the CPU. Also remember when looking at CPUs and Motherboards, keep in mind the appropriate socket and make sure your selections match. For any AthlonXP, you need a Socket A board, for the lower Athlon64s you need a socket 754, and for the highest Athlon64s you'd need socket 939. The same goes if you decide on Intel: most everything is socket , but the newest chips are LGA 775 (sometimes called Socket T).
Anyway, good luck with your computer