The sad thing about selling an older PC is the deals that you can get a new one for.
Now yours seems to have some nice parts on it BUT its not a dual core processor for one thing and the graphics is not that great.
I have to be honest here and say I don't know what its worth as most of the mods you have would only really add to the value for someone that wished these things. I will have a guess but I mean things like all the fans, the fan controller and the case lights. None of these things interest me personally in the slightest and I think many people would feel the same.
You can get a basic 22" monitor for about £140 so depending on make/model and how old it was you MIGHT get £100 if its a newer better one. Failing that maybe £70. For a base unit without all the bells and whistles you might get £150 and if you got the right buyer he or she might pay considerably more.
Sorry but things move on dramatically and you can if you shop about get a dual core desktop with 2 gig of RAM and a 22" monitor with Vista Premium for as little as £400-500. Admitadly yours seems to have better named components but I can't help feeling that most people would not pay any more for it. Just as an example heres a cheap Compaq click here I know its a 20" screen but you get the idea.
Personally I would be tempted to keep all the gubbins and add a new board and processor.