I'm beginning to scope on my next home built ...

So you are upgrading from a I7 920 to a I7 950? Why dont you just overclock the I7 920 to 950 speeds? Same end result!

Why do you think you need to change out the cpu/mobo/ram?
Your i7-920 should perform exactly the same. With a decent air cooler, a 3.3 overclock to the 920 is trivial. Just change the BCLK from 133 to 160 and leave everything else on auto.

I would get a decent SSD for the OS and apps first.

The nehalem memory controller is very good. The cpu is largely insensitive to ram speeds and timings. Synthetic benchmarks can look wonderful, but differences in real application throughput or FPS is measured in the 1-3% range.

As the thread title says "I'm beginning to scope..." I'm not committed to doing it yet.
I'm looking at the state of what I consider to be the "Best Bang for the Buck" is right now, and what the considerations are.

Also, when I do an upgrade for myself, my old one is passed on to my gal friend.
She is presently running my old:
ASUS P5AD2-E Premium
P4 630 - 3.0Ghz Prescott
4x 512GB Kingston [Samsung] value RAM

Whenever I use what is now her's [which is quite often], I keep wanting to kick it hard to speed it up some.
This upgrade for me will move her into the i7 era.
I have OC'd my present i7-920 to 3.0GHz and used it without additional cooling over stock.
CPU core temps stayed reasonable except when stress testing.
At present, it is not OC'd.
I'm assuming the i7-950 would act similarly when OC'd to ~3.3GHz.
Doing it would tell.

It's the MoBo that has peaked my interest in a new build, the Sabertooth X58.
I have run out of SATA RAID connections [I want MORE!].
They are available on the S-T X58 and some are faster.
And, it has 2x eSATA connections compared to only 1 that I have now for the 2 ext HDDs I'm using.
I like the claimed durability for it.
And finally, If I am going to upgrade the MoBo, I might as well do the CPU also, a marginally better CPU for the same price.

I get a new one, she gets a "new one to her" that will be a pleasant surprise.
I win 2 ways.
I get a new PC and she's so nice to me when she's happy.
Are those reasons enough?

I am looking at the possibility of SSD's for the OS drive/partition.
I am presently running the OS on a 75GB Intel Matrix RAID0 drive.
The idea of a pair of SSD's set up as RAID0 intrigues me.
But, I'm not yet sure if it's a feasible plan.
I'm still choking on prices yet.

It is nice to have a place for your old pc. That is my biggest problem upgrading. My kids don't want anything newer.

I am waiting on Intel's "sandy bridge" which is a new 32nm architecture due out in January 2011. I think it will be a significant jump. You might want to read about it: http://www.anandtech.com/show/3871/the-sandy-bridge-pre...

In that same time frame, we will see 25nm SSD's which should be larger and cheaper.
For what it is worth, I changed from two Intel X25-M 80gb ssd's in raid-0 to a single X25-M 160gb. There was no noticeable difference to me in performance. The benchmarks of the raided ssd's were impressive though.

SB is what you should be thinking of updating tbh. I recommend actually like all of them, getting an SSD. I'd recommend the S559 from A-Data, it's great for a budget, plus its faster than the Vertex 2. But if you don't have a particular budget, a good SSD isn't even an SSD, or it is kind of. The OCZ RevoDrive is like 10x faster than an SSD with bigger size too, but it's not cheap.

I would wait for SB/Bulldozer before you upgrade for sure.
This upgrade will give you almost no change in anything, unless the connectors really matter $500+ to you... If you really want to get your GF a new PC an athlon x3 445 should be more than enough unless she's into editing/gaming. Of course, that would be pointless if you upgrade to SB in a few months.