Can you post what the rest of the system is please and include the power supply, The Amps or Watts specific to the +12V rail/s would be good if you know it or just Make and model and we can google it from there. Also what size/res monitor are you using.
The card recomendation really does depend on these things but i would think you are looking at something like the HD4670 or 4770
i think you need a really weak discrete graphics solution to do hybrid, if you have over 500 w you should be able to run a gtx 260, u might have to get a few power converters
------------------------------"Envy is ignorance" - Henry Thoreau : Best quote ever.
i got the OCZ 500W ModXStream Pro Power Supply (http://www.ocztechnology.com/products/power_management/ocz_500w_700w_modxstream_pro_power_supply) , also im in a m-atx and the case allows cards up to 10"
I know that some asus 9800GT (like the one in the link) is hybrid, as are a few 9800GT. Your PSU can easily power it, it can run Fallout 3 and most likely SC 2. It should be well within your budget.
how about this one http://www.ebuyer.com/product/160748 thw X260 is newer then 9800 right, if its worth going for the better card id be happy to compensate the hybrid power