If the Clippers match the offer for Quentin Richardson and the Suns do not spend the rest of their cap space or use it in a trade before next year, then we would end up over the cap due to inevitable extensions(likely with Joe Johnson), right?
Not quite. If the Suns want to exceed the cap in order to re-sign Johnson, they must hold onto his rights. That takes up cap space, to the tune of some percentage of his last (i.e. this) year's salary. I think it's 125%, but I'll look up the source. If the Suns were to have cap space leftover after accounting for Johnson, they could use it on free agents, then use Johnson's Bird rights to extend him for whatever they wanted.
I'll try to look up the source. Stand by...
Edit: Okay, the source is
http://members.cox.net/lmcoon/salarycap.htm#28
And it turns out that, while I had the basic idea right, I was just a tad off on the number. It's not 125% of the last year's salary, it's (ahem) 300%. 300% of Johnson's '04-'05 salary would be $7.1 million, so you'd add that to the team salary of (as things presently stand) probably somewhere around $31.5 million, assuming ballpark $10 million flat for Nash in '05-'06.
And
that means that, if the cap for '05-'06 remains somewhere around $44 million, the Suns would be looking at MLE money -- which they'd have if they went over the cap anyway.
So basically, yeah, you're right: the Suns might as well spend now, as far as the ability to pursue free agents is concerned.