Wait til next month when the GTX's fall in price due to the release of the 7900's, they are stopping production on G70 GT's so you can grab one now for a decent price.
It's really up to you, I love my GT and would opt to get another but I run on a 1280x720 res which is ample for 1 card.
Really look at whats the highest resolution you can run on your monitor before jumping to SLI, a GT will perform nearly the same as a GTX at certain resolutions, you might not want GT's in SLI since it would be a waste of GPU pwoer and money.
You might want to just go for a GTX on a non-SLI mobo
It's really up to you, I love my GT and would opt to get another but I run on a 1280x720 res which is ample for 1 card.
Really look at whats the highest resolution you can run on your monitor before jumping to SLI, a GT will perform nearly the same as a GTX at certain resolutions, you might not want GT's in SLI since it would be a waste of GPU pwoer and money.
You might want to just go for a GTX on a non-SLI mobo