You won't see a major speed boost, really. SCSI is superior to IDE in terms of drive technology and reliability, as well as CPU usage and sustained transfer rates, but it's not something you'll notice on the type of system you have and what you use it for because you're not likely doing any heavy video and multimedia editing that practically requires SCSI. I have a full SCSI disk system (HD, DVD, CDRW) just for kicks--let me rephrase that, I have a full SCSI subsystem because I already had the expensive card and several of the peripherals, and it's cheaper to continue buying SCSI than replace everything--and all I can do is transfer files from one location on my hard drive to another faster than an IDE drive, as well as load CounterStrike maps a full 5-10 seconds faster than anyone else.
If you're not going to use it as a server or as a multimedia editing machine, ignore the cost of SCSI. It's still superior to IDE, but not in any way that would be tangible for you're really that desparate for speed, pick up an ATA/133 RAID card from Promise or someone and set up a RAID 0 array with another hard drive. It would offset some of the problems with IDE and put you more on the level of SCSI, but as a fraction of the price.
Would I see a major difference?
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