Nick,
I have posted a comparison of the X1650 and the X1950 Pro. I know which one I would go for.
When 4.09 goes final you will need a 512mb to take advantage of the long distance views. The Barton 3000 chip was a great CPU in it's time but you should be looking at dual cores and PCI Exp soon if you want to stay up with the pack. Buying a X1950 Pro would tide you over for a while.
BTW, copy that link for GPU review and bookmark it. I couldn't live without it.