The first question is a bit of a matter of application...the i5's only support one thread per core...so some applications will be almost twice as fast on an i7 as they are on an i5 with the same number of cores and clock. That said, there are sandy bridge i7 processors.
I think all the sandy bridge chipsets support dual channel ran. However, I know the Z68 chipset can get a bit unstable when you use it.
They all support software RAID0, I don't think any of them support hardware RAID0.
Well heres some apps I use that are mostly cpu intensive. vitualdub/nandub,.avidemux,vmware for running centos and debian to compile stuff, sony sounforge, behappy, mkvmerge, photoshop sometimes, atm I cant think of others but hopefully thats enough info for a specific recomendation.
