I have that exact card
. It's hard to find something around that price range with performance better than this card.
I don't have SLI, so I can't comment on that. However, it is up to you what you do. Whether it's getting a better single card, like the 7900GS/7900GT, or get two of these 7600GT's.
Let's say you have the choice of either two 7600GT's or one 7900GT. Given these two choices, two 7600GT's are the better choice. Two 7600GT's in SLI are better than a single 7900GT. However, two 7900GT's are better than two 7600GT's.
What I'm saying is, if you get two 7600GT's, it will perform better than one 7900GT. But if you get one 7900GT, then you wait another 5 months and get another 7900GT, it will be better than just getting two 7600GT's all at once.
Keep in mind that I only used the 7900GT as an example, which can be substituted for any card higher than a 7600GT.
So basically I think you have two choices at the moment: Get two 7600GT's
right now. Or get one
higher-than-7600GT card, be patient, and get your second one later.
However, if budget is not a problem then you can just get two high-end cards all at once instead of waiting a few months after your first one.