I am about to by a G4 DP1.25 with 2 gigs of ram and with a 4X AGP ATI Radeon 9000 Pro graphics card with 64 MB of DDR SDRAM. It is running OSX 10.4.11. Will this set up work with Avid Express Pro Mac?

 The GFX card doesn't have enough VRAM for any version of the software above 4.8.4. Requirements after that were 128 megs of vram.

 But that means that I can just replace the graphic card to e.g. an Radeon 9200 GPU with 128MB DDR memory or an e.g. Radeon 9600 PRO PC & Mac Edition for dual monitors? Is there something else I shuold look out for in case I want to upgrade the solution for later like adding a Mojo?

I just saw that Avid stopped selling and updating Avid Express Pro in June 2008 and now merged it into their Media Composer series. And it looks like you need minimum a Mac G5 Intel dual, dual core 2.66 GHz Xeon. So I guess if I go with the Avid Express Pro on this G4 I can never update it to a Media Composer even though I update the graphic card?

You are correct. I have run 'Pro 2.8 on a G4 with a qualified GFX card. Some have even run MC on it but I wouldn't rely on it as my main system were I you.

I'm running XpressPro 5.8 on a G4 DP1.6 with only 1.5 GB RAM, running 10.4.11.  I did have to put a Radeon 9800 graphics card in (the 9600 won't work with my QuickSilver G4). Works fine for me for basic editing, I don't do anything fancy and seldom edit in HD.  I've seen post on the forums from people running MC on their G4's but I haven't tried it since I can't get up to the 2GB minimum RAM requirement.