SATA cards
Its been awhile, but I did that setup - though it is somewhat different:
G5 2.3DC, 2.5GB RAM, X1900, HighPoint SATA card, 3x 300GB in the "front" bay .... 10K/70GB Raptor (boot) and 400GB data/scratch (last two on stock SATA I). It works great - and the only heat issue I've had are in the PCI bays - specifically with the X1900, which was cause in part by dust. The CPU/memory bay is fine, slightly warmer .. and there are utilities you can get that will see some or all the temp senser on your rig ... so you should see the temps in that bay (maybe in incoming air temp and outgoing .. I forget).
Anyway, the setup works great, but my Highpoint card is a bit whack and I had to replaced the X1900 after 11 months of use (my house is a fairly dusty, which is unfortunate). I am looking to upgrade to larger/faster drives... and a better SATA card (or cards). I also have the stock nVidia GF 6600 (re-installed after heat issues) and it seems to help the X1900, I'm guess it catches some of the dust - OR - gives a higher heat reading to the sensor.
I would think just about any set of three, 7200RPM drives would be fine in that bay, though the 10K and 15K would probably stress things to much for long term reliable use.