Can Marc / anyone help please?

My 80gb deskstar Hard drive is getting full quickly! Want to purchase second HD but am a little confused. Can anyone enlightedn me:

1. What is difference between SATA and UDMA?
2. Any notable difference between UDMA 100 and 133?
3. Recommended cache size (minimum/optimum)?
4. Note some of you guys talk about seperate os/boot drive. How does this work? what are benefits/downsides/ how do I set it up?
5. Any recommended buys? Have looked at Hitachi Deskstar 7K250, buffer 8mb, UDMA 100, 160 GB at £72 inc. from Dabs. Any comments?

Thanks in anticipation,

Nigel