Remember that DRM on the PSN Store is not very restrictive, so you could delete the games you don't want to play and whenever you want to play them again you can download them once more.
And you could also just back up these downloads to DVDs or a hard drive, and copy them back to your PSP and activate them again by logging on with the PSN account that purchased it to play them.
And most games for PSP are under 1GB. You can probably fit 20 games on the internal memory alone, add a 16GB M2 stick and that's 40 games. To be honest I don't even have that many games for the PlayStation family of consoles alone...