There would be a bottleneck at some point limiting it.
Probably in memory transfer speed, as a board that old is either using DDR 1 or DDR 2 RAM, ****, possibly even SDRAM.
His memory transfer speeds would probably be much too slow to take full advantage of an SSD.
jbc, give us some more info on your system, so we know what you're working with.
It would help us gauge whether or not this is a worthwhile upgrade.