Wait till the 19th, shortly before this date AMD/ATI are going to release some new mid range GPU's, they will have DX10.1 support.
Search SLI or crossfire anywhere, with a second card, you'll looking at about a maximum of a 30% increase on monitors with resolutions 1920x1200 and higher, any lower and you won't notice enough of a difference to justify its cost.
In a worst case scenario, there are games that are not optimized for SLI/crossfire and it will decrease your performance opposed to increasing it.
Also, high end DX9 card such as this card
, are a fair amount better then low end DX10 hardware, there really is no mid range DX10 hardware at this moment.
You can compare cards on this chart.
Bear in mind, that chart is quite dated (it was last updated in may I believe) and doesn't feature the 8400, 8500 and 8600 series.
Edited by james_8970, 09 November 2007 - 05:07 PM.