I was being a little tongue in cheek when I said it would last a few months
I meant it would keep you happy for a good few months until you want to upgrade again (as newer more demanding games etc. appear).
Of the four machines I've got here at the moment - all are similar spec to the one you are looking at building and I don't forsee a need to upgrade any of them in the near future (though an extra wireless network card could be useful
) Two machines are Pentium 4 based while the other are Athlon XP2500 based. The major difference in performance is due to the graphics card these days.
A good graphics card that would last you for about 18months+ is any of the Radeon 9800XT (or PRO) that are around at the moment. Cost is about £120. They were the top of the range cards until about 2 months ago when the new Radeon 800 and NVidia Geforce 6000 came out.
There are two types of cache - L1 and L2. L1 is built into the chip and L2 is in a seperate chip (but usually attached directly to the processor these days)
Cache is just memory that is attached to the processor and is used to store the program as it's running. This is fast memory that runs at the same speed as the processor in most cases and so the processor can then get the next instruction as quickly as it can without having to wait for the (usually slower) external RAM. As your program is running, it is taken into the internal cache memory - if you have 512K then this means you can have 512K of program ready to run. Unfortunately programs tend not to run like that
and so if it needs a bit of memory that is outside the 512K then it has to wait for the external memory anyway.
Athlons and Pentiums have the same amount of L2 cache these days (512K).
I'd go for the Athlon system.