I believe it would physically fit in the case, but one thing to think about is that the A1133 1.33GHZ board has an internal Airport Extreme wireless card, i.e. it's different than the standard user-replaceable wireless card that your 800MHZ machine has. The internal Airport Extreme on the 1.33MHZ model has two connectors (like a MacBook's wireless) instead of the single connector of the standard Airport Extreme card. And the plug that connects the card to the motherboard is totally different, so you can't re-use your older Airport Extreme on this board. Therefore, if you want to have wireless, you'd also have to replace/upgrade the wireless antenna cabling inside your existing screen enclosure, which means you'd have to remove the screen and hinge, and mess with the cabling inside the hinge, which is not fun. Plus, lots of people who sell the 1.33GHZ board do not include the special internal Airport Extreme it requires, so that's something you'd have to make sure to get. And the custom screen cabling as well.
My two cents is that it's not worth it, unless, of course, you're doing this just for fun. The only practical scenario in which this would even be close to worthwhile is if your 800MHZ is fried, and a free 1.33GHZ board just fell into your lap. Otherwise, it's a ton of work with a lot of risk for not much gain, and you'd probably be spending $200 which should really be put toward a MacBook.