I have the standard eVGA 8800GTX and from what I understand, the 9800GTX performs around 7% faster on average... I don't know how much higher the clocks are on the SC version, but I would guess that performance would be about a wash between the cards.
I'm actually quite disappointed in this 9800GTX launch. 1.5 years pass and 7% is the best nVidia can do? On the plus side, buying my 8800GTX over a year ago has turned out to be an excellent investment, as no other card has completely demolished it yet in performance, and it is still considered a viable card today.
If I were in your position... if you absolutely need a new video card, I would probably purchase a 512MB 8800GT or 9600GT card and wait until summer/fall (When nVidia is supposedly going to launch a 9900GTX part that hopefully will be worthy of the 9000 series moniker). Here's to hoping ATI can put some pressure on nVidia again and jumpstart the stagnant video card market.