Danger den will have the 295 blocks in a few weeks. I am into my water cooling system about 600.00...but I have the largest radiator and eheim pump and two vid cards and cpu...you can do it a for a lot less.
Stable dont know...it will run cooler. I have the i7 block and 4870x2s. I am just trying to keep the temps down. I have done two other systems with water..one with 6600 and two 8800gtx. It runs very cool.
Watercooling is not for the $$$ faint of heart. Trial and error is absolutely the worst method to use when using watercooling as it only takes one error to ruin thousands of dollars of equipment.
No air system can touch the ability of a properly designed and functioning water cooling system.
Another benefit is the quietness of a properly designed water cooling system as compared to having a bunch of fans on the system. I have an "old system" with a 2.66 Penryn water cooled. The system is darn near completely silent. I thought I would try air cooling on my new build, and to be fair it doesn't run very hot on my i7 920 OC'd to 3.6, but I feel like I'm sitting next to a hair dryer after using my water cooled system.
So I ordered a Danger Den acrylic case and the matching system for the case. About $550 total. This way I know I've got a WC system that is designed to be used in conjunction with the case I have. I also bought a MOSFET block and a chipset block so I can cool those without worrying about having air blowing on them.
I will also soon buy water blocks for my GTX 260's, as they are a large portion of the fan noise.
Don't skimp on the system or you will get sub par cooling and none of the benefits of a good water cooling system. Run the system for at least 8 hrs. without the comp powered up to check for leaks. If you don't know how to do that they sell a nifty little adapter at Danger Den.