Well if you have the computer with the 400mHz FSB, then yes, four sticks with 512mb's...However I'm seeing something strange here and I am going to research it real quick.
For the video card, though, you can look here
and look for one that fits your price range.
None of them are too flashy, but they all aren't bad. It's really whatever your budget allows.
The ram you can upgrade to all depends on your bus speed. If you are running pc1066, you can only have a max of 1.5 between the four slots.
If you are running pc800, you can have up to 2GB.
Here's what you should do. Go to cpuid
and download CPU-Z (download link located in the top left corner of the page.) Run that program, and select the "memory tab" (or SPD I think it's called) and you should see a value somewhere in there that says pc****(or pc***), the stars obviously being numbers. Get back when you figure out what speed your RAM is.