Oh! Sorry, I misread it. I've been having a horrible day. There are no Inspiron laptops, as most of you probably know (and were wondering what I was thinking...).
Could it be a Dell Dimension 5150? That's the only other 5150 Dell I saw...
If that's the case, go with this
RAM. It's 2x512MB, so it'll run in Dual Channel, and it should have all the speed you'll need (until you upgrade to Vista).
EDIT: Also, if your budget can accomodate it, you might want to look into getting this
. If I'm correct, you bought your computer with just an Intel GMA in it, no PCI-E x16 (Which Dell's website says it supports) vga card.