What type of music do you play? B3 style organ and/or synth then then on weighted keys are found to be better. But in saying that there are also, "water fall" keyboards! These are a little be better than the lighter plastic ones you can get on some midi keyboards. They are found on high end Arranger and synth 'boards like Korg and Yamaha for example.
On a different note, have you tried placing a thing layer of foam ( about 1cm or 1/4" thick ) along under the key-bed. Using double sided tape, so you can remove it, or change thicknesses. This "stiffens up" the feel of your existing key-bed and may help you out before you lash out with the $$$'s. it also, takes away the "clunk" if you hit the keys too hard. Just a thought...........