I have a G35-AV600 and phone support has no idea what the hard drive can be upgraded to. Not a clue. But they gave me a number to who they said would know: the Parts Depot authorized dealer. I called parts and they said that not only do they have no idea but they can't even sell me an upgrade to my hard drive they can only sell what my computer came with. At end... no one knows anything except hopefully you!
So I pose the following questions I would GREATLY appreciate someone having an answer to. Heck, if the support in the forum are more knowledgeable than the phone support I'd love to pay you guys for this answer! I really need to upgrade my hard drive for more space. This info the entire G35 AV600 community can use. (Personally I am most interested in the hard drive answer but I can see how the other info would be valuable given phone support has no clue)
Hard Drive
1) What can the Hard Drive be updated to? Support told me there are two hard drives in it. Nowadays there are 1TB ready for cheap at 7200rpm. And, if this computer could handle 500gb hard drive would that mean two 250gb or one 500gb, or two 500gb? The bios only recognizes so much. And can it go to 7200rpm?? We currently run at 5400.
Mother Board
2) What is best motherboard our computer can handle?
chipset:
3) What is best chipset we can handle?
Any other upgradeable info would be nice to know 
thank you for any help!
Some answers:
Phone support definitely does not know the answers needed. Tried several times to test if it was the agent or the company.... Confirmed 4 times they have no clue or an idea or who has a clue. So here is what I did find out.
Hard Drive
1) Our computers bios ONLY recognize drive sizes to about 300 GB (600ish total). I had to install several drives to figure this out. What does this mean?
You can buy a 320 GB hard drive and your computer will only see 305243 MB (about 300GB). 8mb are reserved so final grand total will be 297243 (just under 300 GB). So it will be a complete waste of money to buy anything more than 300 GB. EXCEPT:
Good news is we have two hard drives. The bios needs to recognize each drive not the sum of drives so you can install two 320 GB hard drives for a recognizable grand total of 594486 (just under 600 GB). Not too shabby. Currently you can purchase each drive for about $80 so it is economical.
Drive speeds are likely limited to 5400 RPM speed. So no venturing off to 7200 unless you are feeling nutty and want to place an update here after.
If anyone needs instructions on how to upgrade it yourself let me know. It's much easier than you think and will save you money from having a tech do it. If you upgrade just one drive you don't even need to reinstall windows or anything fancy. The second drive allows for hassle free upgrading because it is just storage and does not have windows on it.
Motherboard
2) It's a complete waste of time, money, and common sense points if you try to change the motherboard on your G35-AV600 unless you are swapping it for the same exact one.
Chippies
3) You CAN put a 2.0 dual processor in it. It MAY be possible to place even more but there have been no documented cases yet. If someone has done either it would be great if you share your experience here
RAM
4) 4GB of RAM (two separate 2GB sticks) is just fine. Your computer will show 3.25 GB of RAM being used after you do this. That is a universal 32 bit limitation for all computers. It is worth it to Max the ram as I have noticed a difference on it.
Important information above credits given to people at bestbuy that allowed me to return a hard drive I tested on, http://www.legalissue.com for some tech knowledge and invaluable legal mind work, and all the users here whose information allowed me to cross check things.
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