I was working on one of those awhile ago for my dad's girlfriend, nice little machine. Cheaper than buying the components seperately, and since they got bought out by Gateway they use stock parts rather than the refurbed crap. Looking at it that's a newer model, hers had a smaller HDD, and none of the PCI-E, but it had the same processor, I think, mighta been a 3200.
Keep in mind you'll be buggering up your warranty if anything does go wrong, and they have those fancy new stickers on the back that mean you can't take everything you added out and pretend like you never touched it.
The case should be big enough, and probably has enough ventilation, but I'd advise installing a new PS.