I have the SA8300HD. I did the internal upgrade to a Western Digital 500GB (WD5000AAKB) IDE drive successfully and took some detailed step-by-step pictures to share. Thanks everyone for all the helpful information in this thread!
You'll need a Torx10 Security Bit:
Remove the 3 Torx10 Security Screws from the back:
Then remove the cover:
Remove the 2 regular Torx screws holding the cross-bar and then remove the 8 regular Torx screws holding the side panel:
Slide the side panel out just enough to remove the 4 Philips screws (top) holding the HDD to the top bracket. Unplug the ribbon and power cable. Notice the purple "warranty void" sticker. Fortunately, this can be lifted (carefully) without damage. Someone said that the acid from your finger will change the sticker to "void". I haven't noticed this but didn't look closely either. To be safe, you can wear gloves or wrap some tape around your fingertips:
Slide the side panel out all the way. The HDD is still attached to the side panel, but is now detached from the top bracket. The "warranty void" stick is undamaged. With the HDD out, you can now easily remove the top bracket:
Remove the 3 Philips screws holding the HDD to the side panel. Here is a picture of the original 160GB Maxtor drive. Notice there is a layer of grey gooey substance (perhaps to minimize vibrations):
Here is the original 160GB Maxtor beside my new 500GB Western Digital HDD (WD5000AAKB). Look at your original 160 HDD and notice it is set to "Cable Select". Duplicate this pin setting on your new HDD:
Finally, reverse the procedure with the new 500GB HDD and put everything back together. Hookup your TV cable, your video and power and turn it on. It will reboot several times automatically and in about 15 mins, you should see the clock again. You are now finished. Since the "warranty void" sticker is undamaged, this mod is completely reversible (as far as I can tell).
Disclaimer: This procedure will void your warranty and may damage your hardware and/or software. Proceed at your own risk. Obviously, do not attempt this procedure if you do not own the equipment (e.g. rental units).