Scsi is fast and expensive, but not necessary.
IDE is fast enough if it has an 8mb buffer at 7200rpm
I don't know what Sca is (maybe someone with more knowledge can help)
ATA (?) I'm assuming you mean SATA, because PATA is usually a IDE hard drive. Anyway, it is theoretically faster than the old PATA (IDE) drives, but it's hard to notice. The main advantage of SATA/SATAII is the cabling...it allows better airflow in the case.
Get whatever fits your budget and current set-up. If your mobo supports SATA, go for it. No need to go out and get a Scsi set-up. Hope this helps.