Well first off with the OS, putting a new motherboard on the same windows install is going to leave a huge pile of cobwebbs and is not recomended. Do you have the OEM WinXP CD? Also is it a 64bit version of XP? If it's only 32-bit then you can only use about 3GB of RAM. The rest will just be wasted unless you use it as a cache, pagefile, RAMDISK function.
No the i3-2120 will not bottleneck your GPU. The i3 is a great CPU and will hold up for about anything. It's not ideal for heavily threaded games and apps but it will still perform better than AMD's offerings.
i think my xp was 32 bit, im currently using a cheap laptop that has 64bit windows 7 oem, wonder if i can put this windows 7 into the other 200gb HDD? im guessing not right? also when i got the computer i got 3 cds (computer is HP Pavilion a1524n and i bought it from futureshop)
im fine with 200 gb HDD but il think about getting windows 7 i guess but its kinda expensive XD