IMO, unless some seriously intense apps come out (video editing, some flash games), an overclocked Droid is plenty fast enough.
I feel like a value-added or incremental upgrade would be faster internet, which won't be for about 2years on VZW's LTE network.
I think the X already has the processor and ram in terms of what should last for a good while, at least until your next new-every-two.
The next big leap in smartphones is probably going to be the capability to replace netbooks and low-end PC's, but the hardware is probably 6-12 months away (closer to 6, Droid Pro will probably fit the bill). Of course, that will depend on apps, which will probably be another 6-12 months behind that.
I think the X will hold you over for a good time. Not worth an incremental upgrade, for me, but the Pro will make me seriously consider a 1-yr early upgrade, but it might depend on how close LTE is at that point.