Should I buy another hd 4850 512mb

Getting a second card will improve fps, the number of extra fps depends on your CPU (it has too be fast enough, yours is) and the way the game was programmed (it has to be able to take advantage of 2 cards, not all games do this equally well.)

That being said I don't believe in dual-GPU solutions, getting one top of the line card instead of two so-so cards is often cheaper, uses less electricity, makes less noise, delivers extra fps in all games (no programming requirements) and is overall more efficient: two 600mhz, 512mb cards won't make one 1200mhz, 1024mb card, more like a 900mhz 512mb card.

In this case you could choose between getting another 4850 or selling your first 4850 and getting one 4890 instead of two 4850's, I'd go for the single 4890, unless it's way more expensive than the dual 4850 option.

Also, beware that ATI will probably launch their new 5xxx gpu's this week, that means the price of 4xxx cards will drop a lot (so a single 5850 becomes an option as well, but then again it may not outperform the 4890 by a lot, making the latter a more economical choice.)

Just wait for HD 4890 price reductions and get one of those, a week from now that may be cheaper than getting a HD 4850 today.