The i7 would be an excellent choice IF your client is satisfied with 6GB (I would NOT install more than 3 memory modules since they're unbuffered); and IF he's willing to just have a single physical CPU (an i7 is NOT an "8-core" processor -- hyperthreading lets you run 8 threads & "looks" like an 8-core system ... but it's still only got 4 processing cores => hypertheading does, however, make task switching much more efficient with the extra register sets; so it will give some performance gains over a non-hyperthreaded quad core CPU).
Do you know what his current CPU is? It'd be useful to compare the PassMark rating of his current CPU with the Core i7 920's rating (5450). That would give a much better indication of how much improvement would be likely for the CPU-intensive processing he does.