512 is a good amount of ram, 1 gig is better for sure.
If your budget is more flexible I would seriously consider buying a 9800 PRO. They are quite a bit more, but the performance of the card is what you would want if you are planning on making a gaming PC. Take a look at the difference here on the chart.
http://www20.graphics.tomshardware.c...charts-04.html On the first chart 9800 Pro scored 5966 and the 9600 XT only got 3919.
The 9600 XT will run the games you want, but you will have to turn down a lot of the eye candy. Also, the 9800 Pro will be a card that would be able to keep you happy for many years to come. Why buy something that you will want to replace in a year? That would end up costing twice as much in the big picture. Just wait another few months and count couch change until you can get something that's good, rather than something will tide you over for the time being.