yes, a 2700+ athlon i equivellant to a 2.7Gz pentium. even though the cloc speed is technically slower, this is because AMD Althol chips perform 9 GPU opperations in a clock cycle whilst a pentuim only performs something like 6...
so there is more opperations per clock cycle,
performance for the home user is measure at the most basic level by the processor speed, and front side bus speed...
(I.E Atholons look like they perform lesser)...
however profesionally processing power is measured in Floating point opperations per second.
which is where AMDs seem to accelerate over pentiums, where clock speed is a constant factor...
what you upgrade is really up to you...
however...
Most mother boards have a maximum clock speed, that is to say they wno't take high end chips...
I know that my mother board has the fastest chip it will take, but the cost of the board and chip was les than the cost of the better board, same chip and the a new chip in the future...
and that does still apply if I have to buy a new boardto upgrade...
IMHO, the best upgrade money can buy is definitly more RAM.