Here is a tip:
Don't buy the Sapphire geforce 7900 GS 256mb! I have that card, and sometimes there are just parts of ships missing!
This is a problem with the software of the graphic cards, beceaus it allso is in other games.
But, I think all other 7900GS cards will do.
But, wich connections do you have on your motherboard?
PCIexpres or normal VGA?
And what is your budget?
For €400-500 you have the latest high performance.
For €200-300 you have a high performance card from a half a year ago (wich will be enough for a game like this).
For >€200 you have the latest low budget or mid range cards.
My graphics card costed €215 and it runs shipsim with everything set high and a good framerate.