#1. if you keep updating to new things,like ram, motherboars,processor,would your pc last more than 15 years?
Hard to tell really. 15 years ago, you were looking at AT form factor, or the very beginnings of ATX. Assuming you went ATX, then you could likely still be using the case today, but everything else would have had to been replaced between then. That is of coarse depending on what you do with the computer. Gaming, you would need better graphics, more RAM, better processor, and a board to match it. If you just browse the web, and do office work, then you could likely still be using it still today if you maxxed out the RAM and got the best processor that would fit it.
#2 best graphic card recommended for big games of 2013 like gta v,watch dogs etc.
Depends on how much you want to spend. A 660ti should do fine, but a 680 would be the best .
#3 is 4GB RAM good enough?
depends on what OS you use, and what you do. For gaming, absolutely not. You need 8GB, and you should have 16GB. For basic office work, 4GB is fine. With the right OS, you can get along with 256MB still to this day.
#4 in case i buy a new pc,can i replace the pc's original graphic card,RAM etc with my own that i bought for this computer?
You can use the RAM with no issue. The GPU, you have to ensure that it is compatible, and that the PSU is enough, but it should work.