Basically on the memory side of things...its the CPU features NOT the motherboard, such as this listed under the AMD 955 Phenom II AM3 AS a feature; "Integrated Dual-Channel Memory Controller with up to DDR3-1333 support" and this under the i7 920 features "Memory Controller: Triple channel DDR3 800/1066/1333 MHz".
On the note of Windows 7, it appears the minimum spec is more of less the same as Vista, and Vista works well on 4GB DDR2 anyway. You could always upgrade with another 4GB dual channel kit anyway, I have 4 sticks of RAM in mine at and its still in dual channel. 8GB will surely be enough.
Look for benchmarks in relation to what you want the computer for...I mean for instance the AMD 955 AM3 CPU is better for most games (not much though), yet it isn't as good for most other tasks, compared to the Intel equivalent i7 920. If it was me in this scenario, I would go for the AMD 955. And I should imagine Crossfire (since you mentioned it before) works better on and AMD chipset board too, since AMD is ATi.
For what your looking at in the motherboard/cpu/ram...I would certainly want a better graphics card, but that would require a big increase in budget...I found this interesting article which to be fair has grabbed my attention too...
http://www.tomshardware.co.uk/ ... ossfire,review-31570.html
For the motherboard I would recommend Gigabyte/ASUS and if your into overclocking the DFI boards (since they have every BIOS option you could possibly want).