If you want to keep using your acrylic display, you need an A1006 DVI-ADC converter. These use to be around $50, but have come down in price more recently(I've been able to snag a few when buying otherwise reasonably priced monitors, and paid $30 for another).
Also, assuming your Quad comes with the stock 6600 or 6600LE, you'd be well served by buying a better graphics card. The original "upgrades" were the 7800GT and the Quadro FX4500. The Quadro is the best performing, but is expensive and also dual height(if you care about putting in other expansion cards). The 7800GT is a good compromise and tends to not be very expensive. I recently picked up a retail Radeon X1900GT, which is a great card and was cheap, but these are hard to find(the Mac Pro version is a lot more common but won't work).