I am not sure how the Quadro fairs against a quad (4) SLI configuration but I know that the 7300 GT is definitely the baby of the bunch.
In order I would say:
7300 GT --> 7300 GT SLI (2 cards) --> ATI 1900 --> 7300 GT Quad SLI (4 cards) or Quadro 4500 FX
Bascially the Quadro is a professional workstation card and is not benchmarked for consumer activities such as gaming. It's a card used for professional rendering and is not mass produced as much as the 7 series nVidia cards therefore that is another reason for the high price point.
If you are looking for technical differences the Quadro is a 256 bit card, therefore it will address larger chunks of information in it's memory than that of the 7300 card, which is only 128 bit. However, the 7300 can run in SLI mode, which means there's a interface between 2 cards that allows both of the cards to work together to process the same information, very similar to how the Duo core opporates. Usually for SLI the cards will either process every second line or they will process half the image each (approximately) and show you the combine image on the display.
So to say the Quadro is faster than Quad SLI configuration is arguable. Since I have not seen any tests for this, I do not know if paying $1650 for one card is a better solution than paying an extra $450 for 3 more cards. Think of it as driving 1 BMW 7series or driving 4 BMW 3 Series. Of course a BMW 7 will fit more people and be more powerful than 1 BWM 3 Series, therefore it will do more faster from A to B than the 3. It will probably fit about the same number of people travelling from A to be as 2 BMW 3s but will it compare to 4 BMW 3's?
On this note I'm sure that the 7300 Quad SLI configuration will outperform the ATI 1900 configuration. I just wish Apple would have went with the nVidia 7900 series cards to make this decision easier. 7950 GX2 cards are the highest end consumer cards on the market and are SLI configurable, therefore it would have put even the Quadro 4500FX to shame.
If anyone has an opportunity to test the 4x 7300 configuration vs the Quadro 4500 FX configuration let me know.