Aside from the processor, that seems to be the most asked question in various threads. Perhaps we can assemble the opinions/recs here?
Quote from SawDr:
Thought this might be a good thread to ask this: I have a new Dell with AMD Radeon HD 6450 1GB DDR3 video card - it will run 2 monitors. I need to run 4...is it easier/cheaper to add another card that will run 2 or replace this card with one that will run 4?
Quote from Tom B:
Nvidia Quadro NVS 295 or NVS 300. These are passively cooled (quiet) workstation graphics cards that support two 30" monitors @ 2560 x 1600 resolution.
NVS 295 (256MB):
http://www.nvidia.com/object/product_quadro_nvs_295_us.html
$30 including shipping on ebay
http://www.ebay.com/itm/Dell-NVIDIA...273?pt=PCC_Video_TV_Cards&hash=item2c675da4a9
NVS 300 (512MB):
http://www.nvidia.com/object/product-nvs-300-us.html
$41 including shipping on ebay
http://www.ebay.com/itm/PNY-VCNVS30...356?pt=PCC_Video_TV_Cards&hash=item5895522ba4