I'm essentially in this position.
In the comparison I made for myself (2.26 octo with 12GB RAM in the UK) I found that Apple was more expensive than Dell, but with student discount was slightly cheaper but is much less expandable. I'll probably wait for machines to appear on Dell outlet as they will be much cheaper but I think that won't be for a couple of months at least.
If you can, wait and see what Apple do with the upcoming die shrink (Westmere).
By the time it hits (2010), DDR3 should be dirt cheap, and Apple's R&D costs should be less this time 'round.
If you can't wait, something tells me this will have your name on it some time this week.
(Hint: your name is "Refurbished Mac Pro 8-Core 2.8GHz Intel Xeon")