Upgrading RAM On Inspirion 531S (Dell)?

This is for a friend, I recently did it for my PC.

Here's a helpful website: http://www.memorystock.com/memory/DellInspiron531....

It says her computer can only take

1024MB-667MHz DDR2-667 PC2-5300, 240p DIMM, 1.8v

1024MB-800MHz DDR2-800 PC2-6400, 240p DIMM, 1.8v

Seems to me that her computer should be able to take a lot more RAM. I looked up the info on system, and it said she has 960 MB of ram (of the 1024 MB), which seems rather low to me..

The info on her RAM is Samsung 1 GB 1 Rx8 PC2-6400U-666-12-ZZ & M3778T2863EHS-CF7 0928, so it's 1 GB of PC-6400. Seems decent, I think I should just upgrade the total memory to 2 GB.

Computer is currently running Windows XP Home 32-bit, so the most it could use/detect is 3-3.25 GB. The price of this RAM is pretty cheap, depending on the price I end up paying (more than likely I would buy something through eBay), I could probably get 1-2 additional GBs for $15-35.

The computer is decent, she doesn't use it to run any major programs, but says she was interesting in watching videos (I just recently got her sound to work), and I'm running Opera (5 total tabs), Windows Media Player (Audio CD), and Windows Task Manager. AVG Antivirus, Brother Printer, Soundman, NVidia drivers also in the background. None of that would probably change; with all of that her computer is using 599MB, and the CPU is at 4% (with 8% max peaks).

So, here are the questions:

1. Should I keep the memory currently in there?

2. Upgrade to 2 or 3 GBs?

3. Sell the old memory, and upgrade to a higher bus rate (800 instead of 667)?

4. Brand name memory (Corsair - Value Select) or Generic (Samsung)?

I don't think I have ever seen RAM die in a computer..did when I brought the ram for my PC, I made sure to buy Corsair since if it dies, Corsair will just automatically replace it.

Thanks!