To play planetside well you're going to need a beast of a card. I doubt the gf4 and radeon 9700s will run it with > 40fps all the time (like in battles). So I highly doubt anything low-end is going to run the game decently. You will probably want to consider getting a new system.
If you can't do that now, wait until the game comes out and see how it runs on what cards, and buy off that knowledge.