I'll make two arguments against myself. Maybe it will help you decide... I know that feeling all too well.
Unless you're having heat problems, plan on doing a LOT of overclocking, or have a lot of time and money to kill for little to no performance gain, go with a second GPU. It's a nice thing to have if you plan on showing your system off or will be doing constant benchmarking (again, to show your system off), but it loses a lot of practicality after that. Buying a second GPU will give you an actual performance return for the money and won't threaten your system by literally serving as a sealed fountain. Especially for $250+, you have to ask yourself, is it really worth that much for the knowledge that your system is running a few degrees cooler than it was before?
I'm not entirely unreasonable, though... if what you want is a project, and are dedicated to see it through, then go ahead and buy yourself a water-cooling loop. I'm sure people think the same thing of my building an 80Mhz 486 system with 32Mb of RAM - it's something fun to do that I happen to enjoy, even if it costs a good bit of money to track down every little part. So, if you don't actually
need any more graphical performance in the first place and are looking for something more entertaining to do, a WC loop can be a very satisfying thing to work on.
Of course, that's just my opinion, but you asked for it.