Upgrading Soon

Hello to everyone. I am about to upgrade my own PC (last week I asked for advice regarding a friend's graphics card - I ended up getting him another stick of RAM instead
Flight Simulator and other chat
https://forums.simviation.com/phpBB3/
https://forums.simviation.com/phpBB3/viewtopic.php?f=9&t=23630
Sorry for the long rant, now to the questions:
a) Is it worth it for me to switch to PCI-Ex of that price range? Will the performance be noticable? If Yes: What card do you recommend in that price range?
b) What MB do you recommend for me within that price range?
c) Does the Dual Channel RAM work with all 939MB's and all types of RAM?
d) What if I leave the RAM I have now for a bit?
e) (To someone who has personal experience upgrading from 512mb to 1024mb) Is it worth it? Is this smoothness or just faster loading?
f) Will my 300W power Supply handle this?
Thank you very, very much for any help that you can provide to a very confused guy that is having nightmares deciding what to buy!!!
Vic
Edit: Added power supply info.
I wouldn't mind spending a bit more on the MB, but I cannot really find a 939MB that has AGP ???