While from what I've seen helping people with mods, a better graphics card will help you with things like AA and resolution and perhaps even modded effect shaders where a base graphic is altered in some way. It probably won't help you with things like increasing the number of planets/ships your system can handle, higher resolution textures, higher poly count meshes etc. because the sticking point with these is RAM, not your graphics card.

While the 1.2 patch may have changed things a bit, RAM and perhaps now your CPU seem to be far more important to Sins than your graphics card.

Depends; video cards (at least good ones) have their own RAM. Therefore if the card has more built-in RAM than the current one & the game actually uses it then it would help with these. Especially if it is faster memory (which the GTX 460 is 1800 mHz to his current card's 1080 mHz).

As to the question about buying, if you get a new card it will be possible to transfer it when you do get an new system. Currently the GTX 460 1GB GDDR5 is the one to get; however the next generation of cards will come some time this summer bumping down the prices of current cards. Not sure how much it will help with this game, but as it can be more to another computer later on if you can get a good price then it will be a good thing later on for other games. It is just a matter of when you feel the price is right.

(for more advice on cards, there has been much talk about video cards at http://www.kerberos-productions.com/forums/index.php as many have worried that the upcoming Sword of the Stars II will melt their current card)