Zoltan, first off, I would pull that bar that runs across your video card like Is907 said. That's only there for "structural support", and unless you're using your computer as a load-bearing structure, I think you'll be alright without it. Here are a few ideas you might consider:
1) Get rid of that ugly, green duct. Cut a hole in your case where the processor is and install a cylindrical duct in the side of the case. A few well placed leds can make a fan duct look kick-*ss!
2) Backlight the gray panel in the front of your case with a lazer led. This provides a pretty sweet effect as the light creeps out from the front intake. (I would put the leds behind the Dell logo and point them down)
3) Install a window in the side. Note: If you do this, there's a few things you need to consider. Your case is plastic, therefore, you'll have to use a smaller window to avoid sacrificing too much stability in the side panel. Also, there is a thin, metal sheet on the inside of the panel. With a little creative problem-solving, this shouldn't be a problem. I would cut the metal out larger than the hole you will need for your window, then secure it somehow (electrical tape) so that it doesn't vibrate. Finally, to cut the plastic, use a Dremel with a cutting disk and make sure to take your time. Keep the Dremel speed low so that you don't melt the plastic.
4) It almost goes without saying anymore, but if you are going to install a window, cold cathodes are a must-have, but if you want some other lighting effect (strobes, leds, etc) go for it!
5) Rounded IDE cables. Much cooler than ribbon cables!
These are just a few ideas to get you thinking. If you don't like any of these ideas, you could always put a tie and slacks on your computer and start calling it "Dellbert". I guess that's a mod?! I recommend looking through the mod gallery here and trying to find other things that you might like to try. Good luck, and keep us posted.
PS - Unless you're a Dell Certified Technician, the second you start pulling things apart you void your warranty........just making sure you were aware..........