if i were you, don't buy 600series because you always have to replace them after 2 years (my problem, they don't last for long at high settings, and i see you want to play with eyecandy.) of o were you buy the 9800GTX and later sli it when you think the performance is lower than you want to.
my opinion! but a 9600GT is good, but i think you better go with one card for start. and the 9 series are less powerneeded so i think the question of the psu is YES but it depends of the brand.
The 8800GTS and 9800GTX are pretty close. You can OC a GTS easy to the GTX GPU clock. However I have a feeling the GTX will clock higher. If you are looking for something cost effective, the GTS is the way to go, I LOVE mine, not a single issue. The 2 9600GT's in SLI will be the better performer however you are already SLI'ed where as if you got a single GTS of GTX you would have that avalible down the road. Its your call. What is your budget, what res do you want to play at, what games and do you plan to upgrade at all in the future or is this it for a while?
8800GTS gets my vote. It's the cheapest, performs close to 9800GTX, and gives you the option of SLI later (if you want that).
I think the GTS is your best choice as well. The performance for the price is better and the GTX won't give you much more for a premium price. The 2 9600GT's will outperform both but really do you need that much power? You can get a 2nd GTS when they are cheap if you really need to or sell the GTS and get the next best card at the time.
8800GTS(g92) are already on the EOL list so it might actually be worth it to get a 9800GTX so you could later grab another one when it's cheaper, like in a year from now. However at the same time 9900GTX's are supposed to come out this summer, but you'll still be able to get a 9800GTX for quite some time after that so I'm going to say 9800GTX.
8800GTS is really the best bang for the buck at this time, but I don't think it's going to have the SLI longevity after a year or 2. If you were to consider an 8800GT you could OC that to match an 8800GTS and the when those drop a little more in price you could grab another one and run faster than 2 9600GT's in SLI for the same price.
No matter which way you choose, you will want more power. I have 2 8800GTX ANS3 cards and am always looking for news on the next big break thru and dont even need it. Dont under estimate your psu, its better to overshoot than to under estimate.
Stay focused Grasshopper, too much power will corrupt your ramen noodle...
I will be sitting on my investment for a while. I have a lot of coin I dropped in this system this past month so it is what it is for 3-4 years. The next setup I invest in will make my current rig look like a socket 370 setup if I can help it...