i understand that cutting down costs is desirable, but we shouldn't leave aside the result.
much memory is required for comfortable arrangements and the impulses will need already 4 GB, so 24 GB is a good idea.
usually 24 GB is 6 x 4 GB modules which require 6 memory slots (2 modules per memory channel)
-> remember 2 modules per channel make the memory bus run at 1066 anyway, so you _can_ use DDR3 1333, but you don't need to
the X58 motherboard from INTEL has 4 slots only, but there are X58 (intel chipset) boards with 6 slots (eg. asus or gigabyte s-1366 series)
convolution is a strong job for the CPU, so you may want to consider the 950 (IMO best buy) or 960 (fastest 1066 processor)
-> for the 920 there have been 2 types and i'd recommend the newer model only ... i thought the 920 isn't even longer on the shelves ...
windows 7: professional is a must since the home version has a 16 GB memory limit.
the echo layla (3G?) ... that's a matter of taste IMO assuming the 64bit drivers from echo are ok.
hth, christian