You shouldn't really start 2 threads when they're asking pretty much the same question.
What to upgrade depends on what you use the computer for. If it's general surfing and watching videos etc. then an SSD will work wonders (even though it might not perform at its peak potential on your system). SSD prices have come down a lot in the last few weeks and a 128GB drive is pretty affordable. If it's gaming you're after then upgrade your video card, you have a PCI-E slot so any current GPU will be compatible.
I wouldn't upgrade the RAM though, DDR1 is old tech and very expensive compared to DDR3.
Also what's your budget for the upgrade?