WDC WD5000AAKB-00H8A0 7200 RPM 16 MB cache ATA/100
Seagate ST3250624A 7200 RPM 16 MB cache ATA/100
I didn't know that WD made the Caviar Blue in a PATA model. Live and learn.
Don't think of it as having the two drives go to waste, think of it as a prudent investment. At least, that's what I tell my wife when I buy new gear.
The transfer rates on those drives are 65 MByte/sec sustained for the Seagate and 126 MByte/sec for the WD. That's pretty respectable for the WD, if it's true. The other drive will make a lovely device for backups.
I have no idea, until you test it, if the adapter will slow down that Caviar Blue. If not, the data rate seems reasonable.