Tough call there, man...
But if you're going to sell the white iMac, now is the best time. Apple is likely to update the iMac with the new penryn processor sometime this year (probably in the next 4-5 months at Apple's rate). Sell on Craigslist, eBay or to a friend... whatever. I'd say go with another iMac... if you really want the Mac Pro, I'll be flat honest with you... the refurb is perfect, but by the sound of your thirst for this thing, it isn't going to satisfy your hunger.
I'd say save up for it, get the base line Mac Pro (you can "trick it out" over time), sell your iMac, then upgrade your Mac Pro as you go along. You'll pay the penny for it, but that's the price for this addiction. I say stick with the new iMac, but if you must go, go with the latest and greatest... get the latest Mac Pro.. it's 8 cores and the base line will do just fine for your addiction.