I've programmed in the following:

Kenwood receiver (totally custom)
Kenwood receiver's tuner (totally custom)
RCA VCR (all pre-programmed)
Toshiba DVD (partially custom)
Pioneer LD (totally custom as Option)
Kenwood CD (all pre-programmed)
Digital cable box (partially custom)
Sony LCD projector (totally learned)
Panasonic tube TV (all pre-programmed)

I have yet to see any signs of full memory. In the list above, most of my equipment is being controlled by the pre-programmed codes with some partial customizations. As Daniel said, try using the pre-programmed codes first and then customize/change them from there. The pre-programmed codes, which I'm assuming are burned into ROM, will take up no memory, and the few buttons you may have to "re-teach" will then take less memory to store.

In other words ... go buy it. You won't be sorry.