Putting in a bigger hard drive orig drive is 60 gb or 80 gb

Remove both sticks of RAM. Insert the "new" RAM where the "old" RAM was, and see if your computer reports "zero" RAM or 1GB of RAM. If the latter, put the "old" RAM into the "second" slot, and see if the detected RAM is the sum of the two sticks.

With that model you need a PATA hard drive. You can go as high as 320gb hd in that laptop. Now the higher the GB the more it costs and well that is up to you. I personally would stick with a 160gb 5400rpm drive for decent size and decent speed.

To avoid laptop overheating, follow the information from this link.

You can virtually add any amount of hard drive but I would recommend you to keep it low from 512GB. A 320 GB hard drive in my opinion is a very large one and should work out perfectly.
For the model you can install any company's hard drive until its appropriate to the type you have.
I personally would recommend SeaGate. I have had a very good experience with them.

Hope this helps, Please feel free to ask me for further assistance.

This is  technical process :
what has to be done is the case to the laptop has to be opened (might be best left to a computer repair shop) to which has many steps to it as in 

Keyboard has to be removed 
monitor screen has to be disconnected 
a large number of screws have to be removed 

possibility of Hard drive / ram and a few other devices have to be removed 

once opened the battery has to be located to which is about the size of a nickle or quater that has wires connected to it and a small plug at the end of the wires to which connect to the motherboard 

simply disconnect and then connect the new battery 

now you have to make sure you have the right kind of battery for some laptops take a special battery to which is also totally different from desktops 

Hope this helps 

You get BIOS upgrades from Toshiba.

yeah you need to replace the old hard disk to new IDE 120GB hard disk.

Here is the site I use when I get stuck with repairing Toshibas Good Luck.
http://www.irisvista.com/tech/laptops/ToshibaA75/satA75_1.htm

you can have Toshiba MK1237GSX - 120 GB - internal - 2.5 inch - SATA-100 - 5400 rpm - buffer: 8 MB

this hard drive will work with your consisting config.

find it here :

http://www.sparepartswarehouse.com/Toshiba,Laptop,Part,MK1237GSX.aspx

Could not find sound drivers for your computer but this is the sites searches you might want to try.One of theses sites could also work for the same drivers you are needing.

Toshiba Satellite A75 Driver

Detect Drivers I Need Updates

Drivers For Toshiba A75 S125

Good Luck/Please dont for get to vote.Thank You.