I have no experience with the integrated graphics in the Intel chipset but it should just be a simple matter of putting the new card in, the changeover should be automatic. At worst, you'll have to adjust a couple of jumpers to tell it to disable the onboard graphics. Check your motherboard manual (electronic or otherwise).
As for your copy of XP, if it's a bog standard install CD that Emachines have modified for OEM use you should be fine since you've only changed one component and it isn't the motherboard (it's most likely locked to your particular motherboard baking it impossible to install on another system). On the other hand though, if it's one of those "Recovery Disk" thingies which basically contain an image of the hard disk with everything installed, drivers included, you may be in a little trouble. You might be able to get by with simply reformatting and installing with the CD (with the Intel still activated), putting in the new card, booting up into safe mode and installing hte drivers.