I see - you are concerned the ATA bus will limit the performance of the SDD?
unless your current magnetic HDD is very fast (and loud!) , the speed of the ATA/100 wont be much of a bottle neck. (I get ~75MBps read from my HDD through the 'optical' bus.)
then you could put whatever SSD you're getting on the SATA-bus, and let the HDD reside in the optical bay.
it is actually possible to split the SATA channel, but there are typically severe compatibility issues and you'll...