The chaps over at Madshrimps have compared two Leadtek 9600 GT video cards in SLI against the Geforce 9800 GTX. Here's a taster:

Two cards stacked together do demand some extra electrical current, when you're limited by your PSU the 9800GTX might be a better choice. Heating and cooling are of no issue, the Leadtek 9600GT heatsink has no problem dealing with the heat produced by the G94 core, not even when going SLI. With two cards installed inside your system you might probable notice one card slightly running warmer then the other, though in my testing the mainboard provides a lot of breathing room for the cards and together with the high cooling abilities of my Antec 900 series housing I did not notice any of the two cards running slightly higher then the other. The cards are not too noisy either, they don't offer a superb soundless computing experience but the auto-fan design makes the card rather silent in most classical pc cases.

In the end, price wise you will get slightly higher performance when purchasing a Leadtek 9600GT Extreme. You get the Overlord full game as extra which does add up to the value of the product, if you do not already own the game that is. Whether you should go SLI is another issue, the two Leadtek Extreme samples are factory overclocked but since overclocking doesn't really help that much when using SLI it's pretty much safe to say that a 9600GT SLI setup will most likely perform on par with the 9800GTX, sometimes even outperforming it. Doubling the amount of 9600GT's will get you around the 9800GTX price; you should look around in your favorite on-line store / retailer to see for yourself. The SLI setup doesn't guarantee the extra performance though; where the 9800GTX should have no issue's (in theory) so spend your money wisely.

Leadtek PX9600GT 512MB Extreme:
+ Slightly higher performance
+ Acceptable noise level when being stressed
+ Overclocking headroom
+ HDMI dongle and SP/DIF audio cable
+ PC-game Overlord
+ Power consumption
+ Averaged priced but high/performance ratio

- Does not add much value for money compared to reference 9600 GT

Leadtek PX9600GT 512MB Extreme in SLI:
+ High performance, mostly better then 9800GTX
+ Acceptable noise level when being stressed
+ Overclocking headroom
+ HDMI dongle and SP/DIF audio cable
+ PC-game Overlord
+ Acceptable priced compared with 9800GTX

- SLI functionality not guaranteed in all available games
- Power consumption
- Negligible performance gain when overclocked
- Requires SLI compatible mainboard