Here is what I have found (for what its worth):
I am not sure Xeons are the best.. The Xeons and Motherboards are quite expensive, and you get quite a bang for your buck with i7s..
HFSS is also not completely multi-threaded so you will always spend a lot of time "setting up simulaton" and "building matrixes" so it pays to have a machine which can do this fast (like a gaming machine)
As mentioned before, Memory bandwidth, Memory speed and amount of memory is critical as is a motherboard with fast memory transfer rates.
If you use an i7-920 etc.. you can use triple-channel memory DDR3 RAM. To make best use of this, install 3 DRAMs, so 3 x 4MByte = 12Gig RAM (in the right slots of course)
If you use an i7-2700K or similar, it uses dual-channel DDR3 RAM , so you can insert 2 x 2 Dimms. These support 32GByte RAM (4 x 8Gig) and if you get fast 1600 ram,
you can overclock to 4.2GHz safely. This gives you quite a fast machine quite cheaply. You can buy all this on ebay as a bare bones bundle for not much!
There are even reports of 64GByte machines now that 16Gig RAM is available but I am not sure if this is officially supported.
Incidentally.. look around for the latest 30nm DDR3 ram from a well known Korean company.. quite cheap, very fast and no silly cooling fins, and overclock like crazy.
If course, the i7-3930K or 3960K are slighly better, but not a huge amount.
Incidentlally, the i7-2700K also has a built-in graphics which seems to be fine for HFSS. You do not need a super fancy graphics card.
Oh... one more thing.. HFSS seems to fare much better with hyperthreading turned off in the BIOS. Hope this helps!