Hrm I will try and answer some of these for ya.
First off for raid thats a topic that you could write a small novel on

This is a link to a nice site that gives a whole bunch of info on raid (maybe more than you wanted

)
http://www.pcguide.com/ref/hdd/perf/raid/index.htm
As for IDE and SATA well there isn't a whole lot different between them yet, aside from the snazy SATA cables. As far as performance is concerned SATA in itself doesn't offer a huge bump up in performance over ATA, at least not in its present state.
If your motherboard has onboard SATA RAID and you think you want to use it then a SATA converter would be helpful, since you could use your existing drives with it.
As for Cache well I find that 8mb cache provides a pretty nice bump up in performance. How much performance increase you see probably depends on how you use your machine.
Seek times are a huge deal IMHO. The western digital raptor for instance is a 10k rpm drive with a 5.2ms seek time. There is a very noticeable speed difference over say the 120gb 8mb cache 8.9 ms seek time western digital drives in my other rig(very fast drives in their own right).