IDE drives are slower than SATA drives for two reasons. The big reason is that they are old, and the SATA drives are new, and new drives are faster than old ones.
How fast or slow depends on the drive; you can look up the manufacturer's stated speed for the IDE drive (given the model number) and compare it to that of a recent SATA drive. No one has done a _direct_ comparison, but if you know how fast the IDE drive is and how fast the SATA drive is, the comparison is simple.
Better to buy nice new drives if your budget will allow.
Really depends on the drive. Are they IDE or EIDE? How much cache?
Rotational speed, seek time, latency.
When you can buy a SATA III 1TB HDD for $60.00 it just makes sense to get a new drive. You can always transfer files from the old drives or use them for storage.
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.
Take the others advice here, you don't want to build a new machine running ide drives with some kind of converters on them running into sata ports.
Those converters are usually intended more to help people get data off old drives on a temp basis rather then a long term solution. You could maybe get a few $ for your old drives if you sold them, but ide is pretty much done at this point.