OK, If I can field this one please help me if I get something wrong....
First of all what is the reason that you want to watercool? Secondly, if you are going to be upgrading to a new case, can this wait until you get a new case?
If I was looking for a prebuilt system I would say to look for the Koolance system The Exos system. My brother just bought ond he is OverClocking keeping his temp at 27°C
And that was just for starters. He spent more than $300.00 for the setup.
If you are looking into a new case you can get the Koolance case with all of the water cooling equipment already installed. You will need to install water bolcks and the such but I kind of think that you already know that.
This can get expensive if you are going to buy a prebuilt kit and all of the necessities.
Another good option is to build one on your own. you will need to buy somethings but it is far cheaper to do that, then to spend mega bucks on a prebuilt system.
In my system I have a Heater Core that I just got from Danger Den (I had originally got one from the junk yard - it was very clean).
I have a pump that I got from ebay for 5.00.
Using a res. that I bought from an electrical store... it is one of those square in the ground electrical boxes that has a solid top and seal.
I have a homemade waterblock and tubing that I got from the hardware store.
I have about $60.00 in the whole system. Maybe a little bit more but it is working out pretty well.
Hope this help gives you a little bit of help to bet started with... I would strongly recommend that you look into what you are buying - Read reviews on whatever it is that you are buying BEFORE you buy. You don't want to get something and find out that you really didn't like the features that it carried, or that it was harder to set up then you thought.
-Bri