Don't forget MS live cashback...
Well, although you are switching away from MS, you should check out the MS Live cashback option on ebay which gives you 25% cashback up to $200. So that would bring the $1.800 MP down another two hundred, although you get paid back by MS after 60 days.
BTW, I am a Windows user thinking of switching too. But boy I really don't like the "I'm a PC, I'm a Mac..." add campain; it's sooo annoying and petty.
I myself made quite some research on all the currently available macs. One thing to know about the single quad Harpertown offering is that if you later on wish to add the second processor, you not only pay something like $800 for the extra CPU, you also have to buy a heatsink for it, which is harder to find than the CPU itself. So if you can afford it, the sweetest thing about the Mac Pro offering is that it is literally the cheapest workstation offering out there including a DIY workstation PC. Go to Dell, HP or Lenovo, you'll know what I mean: as soon as you upgrade to two Harpertown Xeons, you get way past the $2.800 mark Apple has set for the entry level MP. So if you cannot afford that $2.800 - which I myself can't - I'd recommend going refurbished or going ebay. That being said, a single quad Harpertown CPU is quite a bit stronger than two 2.66 GHz dual core Woodcrest CPU's. If you look at the barefeets benchmark the link for which has been posted above, a single 2.8 quad core Harpertown is as strong as two 3.0 dual core Woodcrest CPU's. So I'd say either a single quad new computer deal from a retailer or Apple refurbished, or if you can find a good price, a dual quad Harpertown off of ebay is the best way to go for the budget conscious switcher... Mind that if you can't find a good deal on those, still the two 2.66 dual core Woodcrest MP is stronger than the G5.
As for Nehalem. Yes it is around the corner. And yes it has been reported the be a real step up from even Harpertown. Some reports show up to 30% performance increase clock per clock. Nehalem will finally bring about an integrated memory controller architecture for Intel, which was AMD's edge during Pentium days. It will also bring back the "notorious" Hyperthreading technology back to the Intel platform. This technology where two threads can be simultaneously processed on one core, was almost totally useless back in the time when end user software titles were 99% incapabale of multi-threading. Now times are different. So a single Nehalem quad will appear to the OS as an octa-core CPU and software world is more ready for this. So yeah, Nehalem is an exciting thing. But as far as I can see when Apple releases a new revision, it keeps prices pretty much around the same level. So if you cannot afford the entry level Harpertown now, you will not be able to take advantage of Nehalem either.
I know that this post has ended up being mostly about CPU power - but hey I'm a PC and thus I'm a geek! That's all I can know anyways
kf.