My 2 cents:
1) I would wait for sandy bridge. Read about it here:
http://www.anandtech.com/show/3871/the-sandy-bridge-pre...
The tests were done on an engineering sample chip. Actual results will be even better. Motherboards are a competitive market, I expect the 1155 motherboards to be competitively priced. You can lower your costs by looking at a micro-ATX motherboard which costs less to produce. Do you need more than 4 expansion slots today?
2) Initially, the sandy bridge processors will be quad cores. If you want a dual core, the 32nm clarkdale processors, which use 1156, will still be the best. When the sandy bridge dual cores come out, I would expect the demand for 1156 to dry up.
3) 1366/X58 is very good today, but will not get better. It will be a good place to be if you need 6 cores. I expect the follow on's to be oriented toward 6 or 8 or more cores, and ram in excess of 24gb.
4) i7-950 gets you hyperthreading, better SLI, and a potential for 24gb of ram. If these are difference makers to you, then go for it.
SB and i5-760 get you 16gb, 8 x 8 sli which is plenty. I don't put much stock in hyperthreading, unless it is in a duo. Motherboard prices for X58 are higher, because there are more parts to each one.
5) Some applications, like photoshop can make use of lots of ram. If you use one of those kind of apps, then maximum ram might be a consideration. P55 or SB can use 4 sticks of 4gb ram, giving 16gb. If you want more, then X58 can give you 24gb. You need W7 or better to access >16gb.
6) At 1920 x 1200, and considering the cost of the rest of the build, I think a GTX570 or 6870 would be appropriate. Only if you were to game at 2560 x 1600, or with triple monitors would you really need sli.
7) Plan on a SSD for the os and apps. Budget $150-$200. It will be the best performance dollars you ever spend. W7 will take about 13gb of it. Use a large drive for storage and backups.
8) For backups, plan on an external solution.
The value of raid-1 and it's variants like raid-5 for protecting data is that you can recover from a hard drive failure quickly.
It is for servers that can't afford any down time.
Recovery from a hard drive failure is just moments.
Fortunately hard drives do not fail often.
Mean time to failure is claimed to be on the order of 1,000,000 hours.(100 years)
Raid-1 does not protect you from other types of losses such as viruses,
software errors,raid controller failure, operator error, or fire...etc.
For that, you need EXTERNAL backup.
If you have external backup, and can afford some recovery time, then you don't need raid-1.
----------------------------------------------------------bottom line-----------------------------------------------------------
Unless your need is urgent, wait to Jan 9 for sandy bridge.