Hello, I currently have a hombuilt pc with AMD Athlon(tm) 64 X2 Dual Core Processor 6400 , 4Gb RAM, GeForce GTX 550Ti (2Gb), I am contemplating upgrading from Windows XP to Windows7, will there be any loss/gain in performance either browsing the net or gaming?
If you are playing recent games (which support DirectX 10 or 11), Windows 7 will allow you to set graphical settings higher (depending on the game) and possibly even increase performance if the game has optimisations that depend on DX10/11.
Windows XP only has support for up to DirectX 9.
Also, some games run ONLY on DirectX 10 or 11 (Battlefield 3 for example) so upgrading gives you the ability to play those games.
Though your CPU is somewhat old, your system overall looks like it can handle Windows 7 easily. Though Windows 7 may result in a small reduction in performance (system requirements are higher than XP), it won't be noticeable and the benefits outweigh this.
Also remember that there is no way to do an in place upgrade from XP to WIN7 (only Vista can be upgraded XP requires a Fresh install) -- It will require a complete reformat of the HDD and install of win7 - so you'll need to reinstall all of your programs etc. - so be sure to make backups of everything before you start and that you have all of your program discs and keys to reinstall them after the install of win7.