Well, IDE supports rates up to 133MB/s, so that's all you'll get if you use an IDE-SATA adapter. Hmm, thinking about it, IIRC then PCI = 33Mhz x 4 bytes (32 bits) = 133MB/s (also).
If you go PCI to SATA II, I've not found any internal adapters yet.
If nothing else, you'll have an SSD which you can use later on when you've got an interface that properly supports it :-)
[edit]
Regarding your point about the efficiency gain due to lack of seek time, that is true but you may need to factor in a little latency caused by whatever adapter you use. It probably won't be much, but probably worth checking out.
Is your OS 32-bit or 64-bit btw?