The differences are probably fan/heatsink, video outputs, possibly a factory OC and I believe the warranty sometimes varies on EVGA cards.
You don't need a specific brand GTS 250, any will do. If you get a 1gb it will only use 512mb to match your other card however so don't pay extra for it.
As to whether or not you actually need another the answer is it will probably help if your monitor has a high native resolution. At 1680x1050 one should be good but another will help you max out the more intensive games and use higher levels of AA. At resolutions above that it will help more and is a good idea, below that I wouldn't bother because one GTS 250 should already be great.