Dakota, I would highly recommend getting that box down off the shelf! Maybe you should take advantage of that CompUSA deal on Maxtor 160 GB drives I posted earlier, install X on the new drive, and start to become familiar with the OS.
Why?
Well, depending on how it goes, your decision may be quite different than what it would be had you not taken the plunge. You may find that you love OS X and booting into 9 will become moot. Or you may find it isn't your cup of tea, which will demand different HARDWARE decisions. Expensive hardware decisions.
Your cheapest option is over $500 on an upgrade - so playing around a bit first is well worth the time and effort, to avoid costly mistakes.
And make sure that you actually DO something in X - don't just "visit", or you won't become comfortable very quickly. Use whatever freeware and OSX compatible software you can - browsers, Mail, the iApps, AppleWorks, etc. As long as you have enough RAM, your existing machine should handle Jag nicely - not blazingly fast, but certainly not dog-slow! Until recently, I was running X on a 400MHz Sawtooth - which has now been upgraded with a 1GHz Sonnet accelerator. My Powerbook is an 800MHz Ti, and it runs X very nicely indeed.
I took the plunge into X once all the software I currently use was available - I had decided that I was not about to mess around with Classic. After 3 days of nothing but X, I was hooked and had no desire to go back to 9. That was 17 months ago. Others switch back and forth, and some just dabble in X while using 9 for most work. I do know that the only way to become comfortable quickly is to use it exclusively for a stretch, because at first it may be stranger-in-a-strange-land time and you'll be tempted to "go home" to 9.
Anyway, that's the order in which I'd put the horse and the cart...
And if you do decide on spiffy new X-only hardware, I'd follow the advice above and see what the next round of towers brings. Everything I've read points to the new IBM chips appearing soon - by the fall at least, and if Apple is trying to snag the pre-back-to-school season too, perhaps earlier.
