Hi and welcome to ExTS
I found this re. your system:
http://support.dell.com/support/edocs/systems/inspd530/en/OM/HTML/appendix.htm#wp1052310
This will do the job if it has 2 video outputs however - I can't find one for sale.
http://www.amazon.co.uk/XFX-8400GS-nVidia-Express-Graphics/dp/B000SAIQWQ
I did, however, find an 8600GT
http://www.amazon.co.uk/gp/product/B0039EZV4E/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=103612307&pf_rd_s=lpo-top-stripe&pf_rd_t=201&pf_rd_i=B004TRYE5C&pf_rd_m=A3P5ROKL5A1OLE&pf_rd_r=04HDD2J50DXV7H6NMWCF
Now a question for you:
What connections do you have on the monitor cables?
They could be VGA / DVI / HDMI
If they are VGA the card shown has 1 VGA and 1 DVI
You can, however, get an adapter.
http://www.qvs.com/images/DVI-Flat_VGA_ADAP.JPG
One more question:
What is the wattage of your Power Supply Unit.
The 8600GT does not require much - but your PSU must be up to the job.
350 Watts should be fine.