Would it be worth to upgrade from the 800 Mhz to the 1 Ghz?

Here's a complete (to my knowledge) breakdown from incompatible 800 MHz to 1 GHz:

• fan: different connector orientation and cable length. You would need the 1 GHz fan
• heatsink: different shape. You would need the 1 GHz heat sink
• modem: different connector. You would need the 1 GHz modem
• frame: extra screw hole in fan area. You would need the 1 GHz frame

Parts that have different part numbers but I've found to be compatible are as follows:

• upper case
• top shield
• dc-in board

Now for the 1.2 GHz:

• Upper case: has different connectors. You would need the 1.2 GHz upper case
• modem: different modem entirely. You would need the 1.2 GHz modem
• frame: "bump" to accommodate optical cable. You would need the 1.2 GHz frame
• Fan: cut out of top right screw bracket. You would need the 1.2/1.33 GHz fan
• optical cable bracket: more gradual bend in the bracket. You would need the 1.2/1.33 GHz bracket
• optical cable: completely different. You would need the 1.2/1.33 GHz cable

Parts that have different part numbers but I've found to be compatible are as follows:

• top shield
• keyboard
• heatsink
• dc-in board

The 1.33 has even MORE incompatible parts, so no, we can't go back to discussing whether or not you can upgrade to that.

Does this help?

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.

Since you're considering this so that you can run Cubase, you might want to find out if 1GHZ is absolutely required, or just a recommended specification. Have you tried installing and running Cubase on the 800MHZ machine? It may be that it does work on 800MHZ, just slightly slower, and who knows, you may find the speed acceptable. A 1GHZ G4 is really not substantially faster than an 800MHZ G4, and maxing out the RAM and making sure you're running Tiger instead of Leopard (Tiger is more resource efficient) would probably give at least an equivalent boost.

Again, although most of the parts match and it is possible, I'd just hesitate to recommend it due to the cost and effort involved. But if you've done this kind of thing before, can get a 1GHZ board cheap, and 1GHZ really is the magic number that makes Cubase work, then hey, go for it!