Welcome on board, agrats84!
The answer is a mix of yes and no.
Yes, you can move Word, PowerPoint and Excel documents between the Windows- and Mac OS X-versions of Office. However, Windows handles font display, handling, and rendering different than Mac OS X, so sometimes you will find things are not exactly identical. Also, depending on what kinds of content or other components you included in the creation of, say, a PowerPoint presentation, they may or may not totally work.
In most cases it's not an issue, but of course it also depends on just how "creative" you were in the creation of whatever it was you made.
More likely than not, the problems you'll encounter will be Windows -> Mac, and not the other way around. Naturally, the only way to verify this is to actually obtain the software, create something, and then move it over to Windows and see what happens.
iWork is no substitute for MS Office if you'll be trafficking in Office-generated files. That doesn't mean iWork '08 has no place or is otherwise "unworthy" of your consideration. But for the best in compatibility, you need to standardize on your "medium", and if you already know it's going to be Office on the Windows end, you need to be using Office on the Mac end, too.
Consider what you're saying when you say "2.0 GHz or 2.4 GHz". That's two gigahertz, or two thousand megahertz, vs. twenty-four-hundred megahertz. In other words, 400 MHz difference. The old-school rule of thumb is you have to have at least a 15% difference in performance (the new rule is more like 20%+) before you can perceive a difference. 400MHz in this situation is like, what, 16.66%? It's at the absolute bottom threshhold for you to notice. Don't worry about it: you're paying a premium for an improvement that isn't justified by the relatively low-consumption tasks you'll be performing.
If anything, more RAM would be useful. But, then again, that's relative too, since really you only start noticing a RAM-related performance improvement if you're otherwise exhausting the RAM you've got. Generally, I would suggest getting the larger amount of RAM, since even if you were not to take full advantage of it now, more likely than not you will take advantage of it in the not-too-distant future, since ALL software RAM consumption trends upward.
Good luck!
