That's pretty much correct, yes. Each user, from what I understood, has a maximum memory allowance of 100megs per account. PHP generally requires a bit of headroom to operate also, outside of normal processes that eat up memory (a prime example being fcgi), and so I think it's fairly safe to say that the maximum upload size you could get out of PHP on DreamHost would be around 70-80megs. Obviously that number is dependent on many more things, but that's my own experience/statistic with it in a nutshell
Chips N Cheese - Custom PHP installs and the like!