About the card you want to get, I also suggest a 7600. It is cheap and offers the best performance vs. price, and it is a great buy to last you until Dx10.
OR
Perhaps if you could wait a few months, you could purchase a Dx10 card already. You could try the 8800GTS 320mb edition, which could fit your budget. Or, you could wait until ATI releases its line and the 8600 is released, allowing you to get a Dx10 card for even cheaper.