Memory exhaustion & performance tuning

Hello,

First to say I am really an apprentice when it comes to some parts of Drupal even though I run a few (simple) sites on it. I've been cornered by the performance issues recently.

I have migrated from a 64 MB shared hosting to a more expensive 128 MB service, but I hit that blank screen on the module page within two days. I must be doing something terribly wrong! :) I read that CCK, Views and Ubercart tend to be memory heavy, but this heavy? The website hasn't been set to production yet, obviously.

I followed the advice I found like this

1/ I demanded more PHP memory limit from my shared hosting service, from 24 MB to 40 and then 64 MB.
2/ I migrated to Drupal 6 because they say it's less demanding (?)
3/ I'm running latest Drupal 6 now, on an expensive 128 MB hosting and I am stuck.

There must be something more to it than disabling modules. Obviously some modules eat up much more capacity than other, but how do I tell them apart? I'm a CSS/HTML/ecommerce person ans I know very little about PHP. Please give me a hint in the right direction :)