a) Oil is generally not used to generate electricity, so high oil prices do not necessarily push up electricity prices. Gas prices are more relevant, at least in the UK, but while gas prices to the consumer have more than doubled in the past 10 years, electricity prices have gone up below the rate of inflation.
b) The efficiency of the processor is only part of the equation. If you use a CRT monitor with an Iyonix instead of an LCD monitor, you probably use more power than a budget PC with an LCD. Another big saving would be to buy an efficient PSU. A typical ATX PSU is only 60%-70% efficient, but if you pay more you can get one that is fanless and up to 90% efficient. And one can always downclock ones PC in the BIOS, if obsessive about power consumption, and it would still probably be quicker than an Iyonix.
But all this may prove irrelevant. More and more people now want laptops, and there isn't an ARM laptop (A4 aside), nor is there any prospect of one in the future.
Actually the refrigerator is the biggest power consumer in the home. My fridge and freezer are B rated, which I assumed was good when I bought them, but they probably run at around 250% of the electricity usage of A++ rated ones!