The Base

Here's a shot comparing the bases of the SI-120 and the XP-90. As you can see, the base of the SI-120 has some visible machine marks.

A bit of lapping would be beneficial but leaving it alone wouldn't be too detrimental either. My old XP-90 had a distinctly better finish.

Size Comparison

Here are two photos that will show you the difference in size and construction of the Thermalright SI-120 and the XP-90 heatsinks.

Yeah, my XP-90 has taken quite a beating over the years.

As you can see, the SI-120 is both bigger and taller than the XP-90. But the XP-90 has fins that start from the top of base itself.

The Fan

Thermalright heatsinks typically do not come with fans. This is somewhat of a double-edged sword for the newbie but is really a good thing for the hardware enthusiast. While you will have to source for a separate fan, this actually gives you flexibility in choosing the fan you prefer.

Needless to say, it allows Thermalright to sell their heatsinks at a lower price. Since they are targeting hardware enthusiasts, this makes sense because hardware enthusiasts generally have a preference for certain fans and will almost certainly throw aside any generic fan that's included with such a heatsink.

For those who may not be hardcore cooling enthusiasts, Thermalright does not leave us in a lurch. They officially recommend the Panaflo FBA12G12L1A 120mm fan for use with the SI-120. It has the following specifications :

Even if you prefer to use a fan from another manufacturer, it would be a good idea to select one that matches or beats the performance of this Panaflo fan.