Well I will use tbe tower and start from scratch...
That's not a bad idea at all, you already have a decent video card (mid class, 2 years old, fully AH capable) to start with. You might even be able to find a matching second hand motherboard and CPU of the era for next to nothing. The power supply might be good, but being of server type it doesn't have a separate line for the GPU, so that's on the list, too. Using the case and GPU may save you some $150-200 compared to all new equivalents.
Thanks....or maybe i ll buy one of those ebay 500$ pc...
That's not so good of an idea. A $500 pc usually has an integrated video circuit not suitable for gaming. They also tend to be built from cheapest possible components which again are built cheap, so upgrading one with, say, your GTX550, would cause issues in the power supply and possibly other areas, too.