When i type "startx", it says something like 'failed to initialize, no screen found', or something like that anyway.

I'm using RH7, and it detects my monitor correctly (philips 107B), and i'm wondering if then the problem is with my vid card, which is a Hercules Prophet 2 mx.

Any ideas?

Yes it is your card (sorta)
 

Redhat 7.0 shipped with XFree86 4.0.1. Unfortunately, there is no support for the GForce 2 MX with XFree86 4.0.1. What I did was download (in Win98) the packages for XFree86 4.0.2. I copied them over to the Linux partition and then installed them all from the command line. After that running xconfigurator (I think that is the one I used) should do the trick.

Rimmer

PS - Redhat 7.1 comes with XFree86 4.0.3 and works nicely out of the box with the GForce 2 MX