A bit of background first:
The 8086 chip gave a set of instructions - like a dictionary in simple terms - of instructions that the processor would understand. This meant that if the number "90" appeared then it would know to do a "NOP - a 'no instruction' or to wait for one processor clock cycle". This is different from the 68k series of processors used by Apple in that "4e71" was used for a "NOP" instead.
As time progressed there turned out to be a need for extra instruction/words in the dictionary that the chip could understand. These were designed for graphics and sound operations and were called MMX (MultiMedia eXtensions). AMD had their own version that was called 3dNow.
A further addition came with the SSE set (Streaming SIMD Extensions) which were designed for manipulating large amounts of data passing through the processor (e.g. from a video file).
Then came SSE2 which added a few more commands - mostly to do with manipulating bigger numbers.
Each PC based processor these days contains a set of these dictionaries/instructions but the important thing is THEY ALL HAVE THE 8086 SET.
This means software is written using the basic 8086 instructions but can then check to see what processor is included in the computer and chose different/more efficient bits of code depending on which is installed.
The best way of thinking about it is looking at the index in a computer instruction manual. The first page (the index) will be written in one language and it can tell you where to find the correct information in any number of other languages - e.g. Swedish is on page 7, Greek on page 23, English on 54 etc. You can then jump to the correct page and carry on. The information in each section is the same - it's just whichever dictionary the user is familiar with as to which one they use. Windows XP does a similar thing in that it runs the most efficient code for whatever processor you have fitted as it knows they all understand the basic 8086 instructions and can then run the appropriate.
Anyways......
I've got a 3.0G Pentium 4 machine and also a XP2500 Athlon machine here. Both are of a similar standard and both give similar performance. The Pentium 4 processor cost about £150 while the Athlon cost £60. I then overclocked the Athlon to give the equivalent performance to an XP3200 (by changing two settings in the Bios). Personally I like the Athlon processors and most machines I've built for people have had them in as they give the best performance for the money.
BTW1 the latest athlons are the 64 series and these require a different motherboard from the normal Athlons. These are 64bit processors whereas the Athlon XP and Pentium 4's are 32bit processors. You really need a different version of Windows XP for these processors as the normal versions don't have the correct dictionary/instruction set for 64 bit.
BTW2 you are always going to be out of date with whichever board/processor you buy - get used to it
