As for the PC, $800 is more than enough for all your office/multimedia needs (I'm assuming you're going for a widescreen LCD also), an xbox suits your gaming needs better and is easier on your wallet (what with having to upgrade your GPU every 6 months to keep up (which in turn demands better (and more expensive) PSUs)). I'd advise you to steer clear of pre-made rigs, custom-built ones usually end up much cheaper (just mind the motherboard/cpu compatibility).
A mac... really? ( )
Don't you dare belittle XP, SP3 is still the most stable, 3rd party & legacy-friendly OS out there.
Anywhoo, http://www.newegg.com/ is your safest bet.
... I got lost in thought, hope I helped.