I just recently put together a new build 1 1/2 weeks ago, consisting of PII 940 - Asus M3A79 T-Deluxe and I was also doubting towards the end about the maybe still getting the Q9550 instead.
The reason for this were the reviews I read, in the end they all seemed very biased towards the Phenom II 940 and conclusions always degraded the CPU. I eventually decided to stick with the 940 built especially since the obvious price difference and back then I read that intel is moving away from socket 775 (too lazy to search for a link), thus I'm not even taking the risk that I won't be able to upgrade the CPU in a year or two.
If you are going to get a phenom II 940 on a AM2+ (I would advise 790FX and SB750, you don't need an onboard graphics card w/the 790GX) board then you should have an older ahtlon cpu available to update the bios. I ran into this problem and was luckily able to borrow a friends cpu in order to update the BIOS of my Asus board (which you can easily upgrade through a USB stick formatted w/FAT).
My experience with the phenom is very good, I didn't run any benchmarks but I haven't run into any problems yet either.
Regarding crossfire, if your are going to use 2x, 3x or 4x you can only benefit from 4x if your using single slot GPU's, for example on my asus (4x xfire) can only use 3x since there is not enough space to place 4 HD4870's. So you could only use if i'm not mistaken 4x HD4850. But you should check the layout from the board to make sure you can actually make use of mentioned crossfire. Since manufacturers tend to overrate their products.
- Shugs
PS: I paid €222 for the 940 ~2 weeks ago online.