Mostly everything is plug and play for the exception of the CPU which like I said you may have to change the FSB from 100 to 133 being that you are using a 266FSB CPU correct?
The memory is low quality because it's simply PC133, the new standard is DDR whereas that RAM you have is SD
Your board accepts both SD and DDR RAM so you should consider getting DDR RAM that will match the speed of your CPU, in this case PC2100 would match the speeds. Generally I'd suggest buying PC3200 though and it will simply underclock to PC2100 speeds and that way when/if you upgrade you'll be ready with PC3200 and not still stuck way down with PC2100.
MOST instances the PC3200 will underclock to PC2100 but there those few instances where it simply doesn't want to work.
If your mobo is exactly like the one in the pic, then the black slots are your SDRAM PC133 slots and the green ones are your DDR RAM, you can tell by the notches in the socket. 2 notches = SD older PC133 RAM, the single notch = DDR newer RAM (PC2100,PC2700,PC3200)
Reset the CMOS via the jumper and 3 pins, and yes that means you DO NOT need to flash your BIOS.
You will know it's working properly if you go to
My Computer>Properties and it says "Athlon XP 2500+" if it's being seen as like "Athlon XP 1800+" then you know the FSB in the BIOS is too low.