You are way off. The point he's making is, while 4Gb memory is enough for today, the few £ he had to spend to double the memory all the way up to 8Gb would give the build more future proofing. Which it will. What has that got anything to do with P4 vs Sandy? What you are doing is comparing apples to oranges.
Its not way off, while more memory is indeed more useful in 'future proofing' (doesn't exist) .......there was mention above 4 times about getting the 'better' cpu......WHA...................go on, scroll up, loads there about the cpu thing.
Also chummy

iv been messing with PC hardware since 1986 I know what im talking about, im not arrogant I just do.
Putting my money where my mouth is:
The closest cpu's I could find for our little comparison here:
http://www.anandtech.com/bench/Product/65?vs=407
Was the slowest core 2 duo and unfort the pentium sandy bridge.
HOWEVER since the core 2 duo range totally destroyed the pentium 4 class processors, a top of the range pentium 4 would get obliterated by the celeron of today.
Im using this as pentium 4 is easly 10 years old, and help supports my point which is:
'a pentium 640 and a celeron 530 have marginal performance difference, certainly not worth increasing expenditure by 150%'
SO there we have friend, FACT supports my argument, and my hypothesis says in 10 years there will be naff all perceptible performance difference in the 2 CPU's
The RAM as you so sneakily moved onto in your reply (clearly avoiding the above talk about CPU's) would make a more helpful difference, but by that point hard drives will be faster than ram is today, so, meh, tough call, still 8gb wont be particularly useful, I predict it would run about as fast as 4gb on windows 15 or whatever is out.
Now stop dividing by zero and check out the raft of INFORMATION for ones self.
Now sir, I hope one takes this as a humorous post and hopefully be a jolly good sporting chap about the matter, don ones monical ,and retort with wisdom.