I would keep the 512 you have and add 1.5gb to it. You have 3 slots anyway.
And what brands to get.. not really my erea , but crucial , Kingston and corsair are good brands afaik.

does the new stuff have to mach my existing, like if i have pc2100 do i have to buy more pc2100.

Thanks

Even if you have pc1600 installed now , i would still get the pc2100 since the prices are about the same. It will run at the pc1600 speed , but you never know if it comes to use in an other pc in the future. Like i said the prices are about the same.