It looks like Istanbul has its own stepping (D0) when compared to the quad core Deneb parts. There's also a 3.6 GHz Phenom II X4 975 in the cards as well at 140W to replace the X4 965 now at 125W.I've no idea if a new stepping will ship prior to the next model or not in terms of the Thuban. Still, 140W isn't horrible, and it's marketed as a desktop part anyway.
And as you say, it will be easier on the wallet, and there's the potential to undervolt if the board's capable.
P55 isn't going to be replaced any time soon so AMD should aim hard in 2010 with Thuban and the AMD 800 series chipsets with USB 3.0 and SATA 6 Gbps feature sets. You have x4 PCIe 2.0 lanes to communicate down from the 890X/GX to the southbridge. (SB850/SB810)
SATA 6 Gbps is optional but part of the southbridge's native feature set instead of using a third party controller. USB 3.0 looks like a premium feature using their own third party controllers for now. AMD doesn't have plans for native support until 2011 like Intel. Strangely enough though there are going to be H57 "budget" boards for Clarkdale/Lynnfield that have USB 3.0. NEC seems to be the USB 3.0 controller provider for now.
http://www.techpowerup.com/110316/MSI_Readies_First_AMD_890-FX_Based_Motherboard.html
http://www.fudzilla.com/content/view/16372/35/
AM3 + AMD 700 really shined from about May to August when it faced of against mainstream Core 2 Quad. Lynnfield put an end to that in September. Lynnfield and P55 brought Nehalem to the mainstream but the platform's base feature set is limited with the x16 PCIe 2.0 (2 x 8) lanes for graphics and then DMI for I/O. Current AM2+/AM3 owners will see Thuban as a drop-in like Gulftown for LGA 1366. They won't be paying $999 though and even more for the DP versions.
X58 is is a very expandable platform but its features are overkill for a single socket. I still wonder why Intel released it as the replacement to the higher end Core 2 so quickly. They effectively obsoleted Core 2 Quad but never changed the pricing on it. It's a DP workstaion platform that launched as a gamer's one first.