<<what is the difference between IDE and SCSI>>>
Unfortunately, I dont really have a spare 3 days to explain...
Suffice to say, it's a different interface - has less overhead because the commands are processed by a SCSI processor rather than the system's CPU.
IDE doesn't have ah heck all overhead since the implementation of DMA, but still more than SCSI.
Also, SCSI has the advantage of superior daisy-chaining, multiple types of devices (scanners, tape, HDD, even floppy), and, of course, speed!
ATA133 isn't quite as good as Ultra320...
Go SCSI!
High FSBs ?
Well, Avansys cards dont like it. If you can keep your PCI bus speed down, then sure, but the higher speeds stop the BIOS from intializing in most cases.
Without onboard BIOS, any scsi card is likely to handle at least a slight increase in bus speed.
Adaptec cards love it, and although my PCI bus is 44mhz (133/3... yes, /3), my HP Netraid controller works fine.
So basically, get a high-quality card, and you wont have problems.
(BTW: I have seen avansys and tekram cards bring a system to the speed of your average dead camel...)