Please also be reminded of scook's post #3 - if you install 32 bit Windows, your system will only see approx 3.25Gb of your installed RAM.
 
The remainder will sit there doing nothing.

Yeah, I definitely plan to get 64 bit Windows 7.  If the package happens to contain both a 32 and 64 bit version (like another poster suggested), then that's fine.  I know 64 bit will run on my system, the XP I was running was 64 bit.  Whether Windows 7 will run or not is another story.  We'll see what happens in the next day or 2.  I've been watching the X3 videos on Cake TV and YouTube and am getting pretty jazzed!  Thanks for all the help.

Update:  With the price of a retail version of Windows 7 being over $200, and with my PC being 5 years old, I starting leaning toward just buying a new system with Windows 7 pre-installed.  Then I found that I could get 24 month no-interest financing from Sweetwater, and that they sell custom-built systems optimized for audio.  So next week sometime, I should be getting my new system, and can then finally start working with X3.  The specs are as follows:  Intel Haswell Core i5 Quad 3.4Ghz, 4GB RAM, 2x 1TB HD, Windows 7 Home Premium 64 bit.
 
I know 4 GB RAM is pretty slim, so I think as soon as it's delivered I'll add another 4 GB.  The other drawback to the system is there's no Firewire port; it's all USB.  My current audio interface is Firewire, so I ordered the Focusrite Scarlett 2i2 USB interface for use with the new system.  As 2 simultaneous inputs is the most I ever record at one time, I'm not too worried about the whole USB vs. Firewire issue.
 
As someone who primarily uses synth and drum plug-ins, with the only live instruments being guitars and vocals, I think this setup should serve me pretty well.  Yes, I could have taken the time to build a comparable system for less money, but that's a little out of my comfort zone, and I mentioned the no interest financing!  I'd welcome any thoughts or words of advice/caution about this system and how it should handle running Sonar X3.  Thanks to all!

Exactly. If that is the case go windows 8.1, note upgrade from 8 to 8.1 is free.

Well, it's already ordered with Windows 7 64 bit, so nothing I can really do now.  Everything I've read indicates that it's a very good OS for DAW work.  With the general uproar over Windows 8 when it came out, I thought that going with 7 was a wise choice.  I know 8.1 was supposed to have helped a lot.  Are you happy with Windows 8.1?  And do you see a big advantage in using that over 7?

Don't sweat it - Win 7 is an exceptional platform for running DAW software.