My currents specs are: Asus P5N-D 750i Sli (2x Pci-e x16 2.0),Core 2 Duo E8400, 8800 GTS 512,2GB ram 800 mhz, 600 watts power supply.Is buying a second 8800 GTS a good Idea? I saw some reviews with 2 x 8800 GT's beating one GTX280 and 2x8800 GTS are better than 2 GT. Is buying a second 8800 GTS 512 a good option ?
It's a very good option, particularly with 800GTS prices what they are now. Alternately, check reviews and see if there's anything about how the 4850 or 4870 stacks up against a pair of 8800GTS in SLI; for that matter, just wait for the July version of "The Best Graphics Cards for the Money" (and read the feedback from other posters).
The 4850 probably won't beat SLI 8800GTS cards, though the 4870 might, and the 8800GTS are almost certainly cheaper.
The 4850 does beat the 8800GTS, and even the 9800GTX. However, if you already have an 8800GTS and a SLI MB and a good PSU, your best option is to add another 8800GTS indeed. 8800GTS SLI will beat the HD 4850 big time, it even beats the GTX 280.
What kind of PSU do you have? Not all 600W PSUs are equal, you know...
If you play at resolutions/quality that can take benefit of SLI, then yes, a second GFX would be the most noticeable gain for the price. Just make sure your PSU can handle it. If the games you want to play take too much RAM (I know Crysis takes a decent amount), getting another 2GB RAM could remove a possible bottleneck.