erm i just upgraded my gpu
right now is ati's 4850
can someone recommend the best GSdx for this gpu?
Use the latest version in the public beta, unless you want to compile later versions yourself.
It also depends on what your CPU is, more specifically what instruction sets it uses (ie. SSE2, SSSE3 [note this is not the same as SSE3] or SSE4.1). Use the appropriate GSdx for the instruction set that your CPU uses (it should say which instruction sets are detected in the console window when you run PCSX2).