You might look into the Logitech Trackman Marble. It's a USB device, but has an adapter to PS/2. I use one on the 2000 and two 2480s. You would still,have the wire, but the device is stationary which can be a real boon when surface real estate is in short supply. Another point in its favor is no need for batteries!
If you use a Y adapter to connect mouse and keyboard, be aware that the plugs may be reversed; use the mouse plug for kb and vice-versa. Maybe its just my adapters, but all three are that way. It ws a bit consternating at first.