That board goes to a max of abut 450FSB.
That then equals 7 * 450FSB = 3150MHz.
That would be about the max you could achive.
You wont see 5GHz with water thats just stupid.
For evey 10C you lower your CPU you get about 1-3% more oc.
It actualy depends on CPU you could get nothing actualy with water. It's just important to keep your temps down so you dont risk your CPU oveheating or shortening its life.
Tuniq Tower will be just fine, if you dont up the voltage too much.
I would suggest you to get the 6400 it's just a bit higher priced and has a multyplier of 8 meaning it could go 8 * 450 = 3600MHz.
Overclocking is no sure bet, no one knows what you will get.
Maybe you get it to 2.5 or maybe you get it to 4GHz I wouldnt expect it above that (You would need a sub zero cooling like liquid nitrogen to get it to 5GHz).
BTW I'm getting an Asus P5W DH Deluxe (probably the best mobo out to OC a Conroe)
It is a good board for ocing a Conroe.
But you have an Allendale - and for them, the Gigabyte DS3 and DS4 is better.
Check xtremesystems, you'll see many hit 450+ with that board; the highest (though absolutely unstable) was 535x4=2140 MHz FSB 8O