Is it possible to get the graphics card of my iMac to my XP?

It would work, but not right away.

Hardware in Macs are designed specifically for Macs.

You'd need Apple written drivers for your Windows computer. Google search "Bootcamp Windows Drivers"

iMac does not have any PCI cards. Only towers have PCI cards.

If you have the 2.66GHz iMac 24", the graphics chipset is part of the logic board.

If you have the 2.93GHz or 3.06GHz 24" iMac, it had the option of three different MXM video cards from the factory. These are all separate cards with separate video memory. They are essentially notebook video cards, although nowadays, almost all notebooks use either integrated graphics (part of the main processor) or on-board graphics chips.

Other iMacs are similarly fixed: either on-board graphics or MXM card.

If you thought to pursue this idea further by removing an iMac MXM card to use in a PC all-in-one or notebook, it still likely wouldn't be useful, since the Mac firmware is totally useless in a PC, and vice versa. Although a few flash options are available for specific PCI or AGP cards, I know of none for MXM cards.

if you got that PC in 2005, chances are that's it's old scrap of junk. So even if you manage to get the ATI Radeon out of your Mac to stick it into that PC... it probably wouldn't work anyway due to the crappiness of the psu.