You can't use a 120mm fan on the SLK-900. The largest size that you can use (without some NICE modding) is a 92mm. I am building another system in a few months and I love the SLK-900. I have it in my current system that I built a few months ago with an 80mm fan. In the new system I am going to use a 92mm fan.
And yes, they attach by fan clips. The SLK-900 can also be used in Pentium systems not just AMD. The box supplies clips for both Pentium and AMD's.
Take a look at Thermalright's website for more information on the SLK-900.
http://www.thermalright.com/slk900.html
For the high speed system you are using the SLK-900 would work great. It is the best performing heatsink available right now.
Note that one problem you may come upon is if you want to install a fangrill on the fan that is attached to the SLK-900. This can be a pain to do. I managed to get a fan grill installed on my 80mm fan and finally get it clipped onto the heatsink. It took a lot of work though. Also be very careful when installing this heatsink.
You HAVE to install this heatsink (on AMD) systems by screws into the motherboard. With P4 systems you can use the fan mount on the motherboards, I think. You MUST be very careful when installing this heatsink or you can very easily crack your processor. Do not apply much pressure to the processor when your tightening the screws. Turn with minimal force and when your are unable to turn it anymore with that small amount of force it may be a good idea to stop right then. Also, make sure you DO use the pads on the processor that are included with the heatsink even though your processor already has the circular pads on it, they are not usually the right height for the SLK-900.
I got by with not doing it but I am a bit more experienced so trust me, it is better to be safe than sorry.
Oh and make sure you use thermal paste. Artic Silver III would be a good idea.