Then use PCI Express or PCI X. I have several 64-bit PCI devices, Gigabit LAN, PATA, SATA, and SCSI.
However, the limited bandwidth of 33MHz 32bit PCI isn't that limiting. Use various methods to distribute load by combining PATA, SATA, SCSI, USB, IEEE 1394, LAN, etc. If your CPU isn't powerful enough for your tasks, get dual-core Pentium D 820 or two Xeon 2.8GHz. Have more RAM and more powerful PSU.
Big case isn't as important as there are quite many cases for holding several optical drives inside and they usually cost under US$50 and optical drives generate little heat unless you plan to torture everything on a PC by burning DVD write-once media at 16x in several drives at once for 24 hours everyday.
Problem is the PCI Sil0680 seems to be speed-limited by the PCI channel anyways, so using 2 channels provided by the one card at the same time gives the same see-saw problem.
PCI 32-bit 33MHz provides 133MB/s which is enough even for four channels at most times. First define what your needs are.
We want to have 6-9 DVDR drives.
Too vague. You can have 50 DVD writer drives in one PC if you want.