You never know what's round the corner
If you can afford - go dual core - no such thing as future proof ... but you'll get a couple of extra months.
The whole speed thing was (is) a total con which was why AMD changed their system. The actual measure of "speed" is MIPS
http://en.wikipedia.org/wiki/Million_instructi ons_per_second
(and even that is up for discussion)
A PC is like a model road layout with a lorry carrying boxes from A to B .... one section (the processor) is 70 MPH ... another (PCI) is 40mph, (IDE) 20mph etc etc.
Upping the processor to 100mph looks good .... but actually may only result in a 5% increase in the number of boxes carried overall.
Put in a second road and even if you reduce the speed limit .... overall you will still see an 80% increase in the boxes carried.
Unfortunately ... you still have to get into the warehouse to store the boxes ... so the speed comes down further.
Graphics is mainly about shifting boxes ... so benefits immensely
Converting sound is about wharehouse space ... so not as big a gain.
We'll leave heat issues for later ... but its about track thickness