You can somewhat use SpecINT benchmark numbers from spec.org to guide you.
As a general rule, you can divide the speed of the Intel processor by about 2 to get the equivalent Sun CPU speed you need (provided each Sun CPU has at least 2MB of L2 or L3 cache). This is not an exact comparison but takes into account some of the other features that Sun systems have that make multitasking OSes run smoothly.
There are however a few considerations like memory bandwidth, expandability and max RAM capacity that are also important depending on your needs.
However, the real question is, what OS are you going to run?
Solaris has a lot of nice features, but if you are going to use Linux, I would suggest sticking with x86. If you care about Solaris, buy Sun, or at least SMP x86 systems that are on their HCL (hardware compatibility list). Solaris performs very well on SMP.
