The bus interface does not make up for speed limitations of the drive itself. I mean, that's why ATA133 was kind of a joke when it came out, because drives were still having trouble pushing ATA66 to it's full ability. (let alone ATA100)...
SATA based HDDs were subject to the same issues when they first came out. While the bus speed was faster, the drives themselves rarely outperformed their PATA equals. These days are a bit different, with drives like the WD Raptor, but as a general rule of thumb you buy into SATA for standards compliance and upgradability... not blinding speed differences.
But it's up to you, I'd say if you are using it as your main computer, go for the upgrade. But even with the upgrade, you'll get much better performance out of an external firewire drive enclosure, and a new (modern) desktop sized hard drive.