It has a 1.5gb/s SATA interface, so probably the cheapest will do fine. The great modern drives like Samsung 840Pro and OZC Vector push the 6Gb/s SATA 3 limit, and that is complete nonsense on this old interface. It might even give you trouble.
Why not pop in a Scorpio Black 750Gb? They are cheaper than all 120Gb SSD's, and at 1.3 Gb/s they already push the interface.