I would say that a good beginner setup will run you at minimum $200, more than likely for CPU only. If you start out with decent gear like a strong pump, 2x120mm (minimum) radiator and decent fans, you can easily add a GPU block and do fine. I am cooling my Q6600 and it has been running like a champ, but your choice of hardware is up to you. I personally don't know the limits of the Q9xxx series, but I do alright with the Q6600's (as well as many, many others on THG forums are with the Q6600...I'm sure there is a strong Q9xxx following as well).
What is your intended budget? What kind of OC are you looking for...mild or high? What are your expectations, reservations or fears?
There are several regulars around that are very knowledgeable and can suggest some great ideas...my suggestion is to stay away from Thermaltake, Koolance (in most cases) or any of those 'bay drive' water kits (less than 400 Liters/hr; the Swiftech MCP355 is an exception at ~400 L/hr). Check out
www.frozencpu.com for Swiftech or Enheim pumps, Danger Den or Swiftech blocks, and various other liquid supplies. They have some starter kits, but you can just as easily piece a kit together. I know Newegg sells a Swiftech kit:
http://www.newegg.com/Product/Product.aspx?Item=N82E16835108076
that is a great starter kit. You could easily add a GPU block to this and maybe even a northbridge block if you didn't get too insane with your overclocking.
Temp decrease? I would say you should expect to run temps at load what you might see at idle with air cooling..depending on current cooler and chosen water system. As for GPU...expect to see about 1/2 temps at load (80C -> 40/45C)