You only need a motherboard thats compatible with your RAM and CPU a motherboard won't matter to software. Dont think any motherboard exists with a sound chip that can do ASIO so get a dedicated soundcard if you dont already have one. Whats your other components have you got a fast enough hard drive and will your power supply unit be enough?
The i5 is faster but the AMD phenom II x4 will do the job easy.
Also get a third party CPU cooler those i5 or i7 overclock so much its best to buy the slowest one then OC it.
For ram get 4 or 8 GB DDR3 Corsair Dominator. http://www.corsair.com/products/dominator/default.aspx
Lower latency is better. Faster ram is better than too much ram. Omnisphere will be limited to 4GB of ram if its running in a 32bit DAW Windows 7 will use over 1GB before your software loads so prob best to get 8GB. Make sure you install Windows 7 64bit as the 32bit version is limited to 4GB of RAM. Win 7 64bit can use 16GB.
I built my Core i7 computer with this stuff very stable runs Omnisphere and Trilian really well. My friend built his computer with the AMD Phennom II X4 and is also compatible with Corsair Dominator RAM.
His PSU died after 2 months though it was overstressed and a cheap one so he replaced it with the Corsair 750 which is what you need if you also have a monster graphics card in there.