I'm thinking 3 9.1GB drives. This, if I understand it correctly, would give me 18GB of storage and one drive for parity

Correct. Though the parity is spread across the 3 drives.

These are the drives I was thinking of getting, as to not hit my wallet too hard.

50 pin? Thats going to probably indicate those drives can only pull a maximum of 20mb/s, and probably closer to 10. My 30gb IDE can pull 27.2 real world.

Go with IDE RAID for a PC. IDE raid cards varry on price, any IDE card that can do Raid 5 will be around $200 minimum, but it will be a decent card. To get peformance with SCSI, it costs money. IDE for a PC use will be fine.

one side question for RAM. what is ECC and is it better with or without?

Depends. ECC is an error correcting scheme for memory, allowing simple failures to occur without impacting the machine. Normally a memory error crashes a machine instantly (OS typicially crashes it's self to protect data from furthur harm). ECC is used in almost all servers. As long as your motherboard supports it, and provides some warning when errors have been fixed, I'd say go for it. But if it can't notify you when errors occur, it's not very useful. ECC can generally give signs it's about to fail, and allow you to replace the module before it craps out completly and leavs you with odd blue screens or kernel panics.