Varborta,
There are some nice round-up reviews on Anandtech, Firing Squad and Tom's Hardware, but without having more info about what your needs/wants are, it's kind of hard to answer your question. Although each manufacturer offers models with variations on TV-Out, S-Video and Dual-display options, none of the makers have deviated much from the Nvidia reference design as far as the basic guts of the card.
HOWEVER, there are differences in memory. For example, Hercules puts 5.5 nanosecond chips on their retail card (the OEM card has 6.5 or 7, I think), whereas other manufacturers are shipping with 6.5 or 7 ns memory chips. This will be important if you plan to overclock the card, as OCing the memory will get you the biggest gains, and the 5.5ns chips are rated to run at a higher frequency than the slower stuff.
Hope this helps...