I have a quadro fx5800 & a GTX 480
The 480 is a great card for the price.
Till now, everybody is guessing, because there is no comparison material available. I have chosen the 480 and hope that the extra cores will pay off in performance.
According to the CUDA scores the 480 is higher (480 over 192), but the 3800's clock speed and memory is much higher.
The 3800 clock speed is much LOWER than the 480. Spec wise the 480 is the clear winner on all fronts.
I would use the savings of not buying the 3800 but the 480 to remove your system bottleneck by getting some additional disks. Only 2 HDD's is a serious bottleneck, and do check your PSU if you decide on the 480. 750 W looks marginal.
I completely agree with Harm and Baz. If I were you and had that ~$300 and could wait for a short while I would go with one of the new WD VelociRaptor 600 GB drives for the OS and applications. With the new 32 MB of cache and apparently higher density (faster read and write rates) it should be even better than the old suggested OS drive (WD VelociRaptor 150 GB).