Yeah, good choice. It'd definitely be faster than the x600 you were going to get
Yeah, the Leadtek seems the best out of all, and also the clocks seem to be rated higher than the Abit, or Sapphire versions which is a bit weird (Leadtek to my knowledge doesn't pre-overclock)
Also if you wanted to run SLI in the future (would need a new motherboard if yours doesn't support it now) then the Leadtek is also a good choice for that! (Basically if you don't know what it is. It allows you to place 2 video cards into the same system and allows games to run faster), and the Abit and Sapphire versions also doesn't allow for it
Everything else though about the cards are the same because they are still from Nvidia.
The only thing that will be different then will be the noise of the fan, and I have no idea without hearing them how loud they will be.
You also get some games and software free which the others don't! Leadtek is usually known for doing that
I hope this helps