Hi,
you say you do lots of music and video editing.
I would recommend the 7800GT Mac Edition. It is quiet the same as the quadro FX 4500.
The only difference is, that the quadro was optimized for people who do CAD (do you have the same abbreviation in english?). I mean people that construct heavy machines on their mac that are built in the industry later.
If you do music and video editing, the 7800GT 256MB Mac edition is somehow the "version" of the quadro for you ;-)
But why do I recommend it? I do not know the prices on US-ebay, but in germany you get a 7800GT Mac Edition for 100-150EUR and a
PC quadro FX 4500 not flashed, costs the
same plus you have the trouble with flashing it. (A 7800GT PC-version unflashed costs about 30EUR). Well ok, honestly the 7800GT will behave like a x1900, and the quadro FX4500 will be like a 7800GTX 512MB (the latter never been available as Apple Version, but you can flash it as well).
Though an x1900 would be plenty enough for music and video editing. Did you just try doing your works with the 6600?
Read here for a comparison and to see a hirarchy
http://www.jcsenterprises.com/Japamacs_Page/Blog/71BBF3EF-9713-4C53-8B80-26771F8A4087.html