The western Digital Caviar Special Edition series is hands down the fastest IDE disk out there. With it's 8Mb buffer it can archieve speeds that are equal to (slow) 10.000RPM SCSI harddisks in some tests.

It's very affordable too and is available in 80,100 and 120gb sizes. (see here)

I have one and I've been very impressed with it.
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Riocar 80gig S/N : 010101580 red
Riocar 80gig (010102106) - backup