What are the operating limits of windows xp professional

What limits do you want to know? Memory? that is limited to 4gigabytes of total addressable memory. Video cards with dedicated memory will come off the 4 gigabyte limit,

I would never drop from 64 bit to 32 bit on Win7 even if you had 8-16 gigabytes of memory XP would just not use it and at least 7 could when it was running.

First of all, if it came with 64bit you were already licensed for 32 bit. You should not have bought it.

Secondly, there is almost no reason. Not to use 64 bit, and if you have 4GB of memory you ABSOLUTELY want 64bit or you won't be able to use all your memory. 32Bit should only ever be used if a specific piece of critical software is incompatible.

thirdly, why are you asking about XP when you said you installed 7?