I went with a WD 160GB 5400rpm on my old Dell 8600 (ATA-6) to replace a dying 80GB. It's a little quicker, but nothing to write home about. The rest of the system is just too slow to take advantage of any speed increase. It was only about $15 more than an 80GB ($70 vs $55).
I don't think I saw any 7200rpm ATA drives, but I could be mistaken. The biggest reason i think to change would be the increase in storage space or, as in my case, to replace a dying drive.