nothing aside from clock speed differences if they're factory overclocked, and choice of colors for PCB, and or coolers, some odd ball manufacturers like to redesign the layout of the boards making them shorter, but thats not with all cards/chips just some. haven't seen any 4830's like that though.
best thing to do is search for price, and keep an eye on what length and type of warranty the manufacturer has.
generally XFX, Evga and BFG have the best warranties, but varying prices, and clock speeds (Core, Shader & Memory clocks) on their cards. (though don't know if EVGA or BFG currently have any ATI cards in their lineups?)
MSI usually offers good value for the money though.
for ATI cards, Sapphire and PowerColor are two of the older more seasoned brands for ATI cards and have a fairly loyal following, though XFX last year started to carry ATI chips as well, instead of exclusively just Nvidia. which opened up new possibilities for some Die Hard XFX fans.
not uncommon for XFX cards to carry a Double Lifetime Warranty.
if you can afford It I'd say go for the XFX cards, if you can't next best bet would be Sapphire, Powercolor or Gigabyte (pretty solid brand as well), and then after that Value wise, MSI.
hardware
prices 
