Whoops, I just caught your mention of price.
With just $100 to spend I'd definently recommend RAM.
$100 won't even come close to buying you a video card that would come close to providing a significant impact on your gaming experience.
$200 would buy you an alright card, like a GF4 4600 or ATI 9500, but anything less would hardly even replace what you already have.
On the other hand, $100 will buy at least 2 sticks of 256MB DDR RAM.
