I'd go with the PSP games. I'm sure that somone will come out with hacks. They are already working on a sytem that works off the memory cards. Plus, Sony speculates that the later versions of the PSP will probably include email,voice chat, word processing, and even have talked about upgrading the media player.
Avoiding the updates is going to be too much work and not enough fun. Thats what you bought your PSP for, right? fun? We all remember FUN. Like when we gave our friends some sheep milk replacer just to see them gag.
You looked for hacks for a DS? You must'ev been desperate.