I have a 1650 by 1080 22inch monitor and a 8800GTS 640mb card. I only have a 550w PS so I would have to upgrade PS. Should I go with another 8800GTS card in SLI or another card and use my old 8800GTS for PhysX?
I have heard you can use a Radeon Card and dedicate the Nvidia card to PhysX.
Or would a SSD be a better upgrade?
No, it's not worth it to SLi another 8800GTS 640mb.
You have the 8800GTS based on the old G80 architecture, so your card performs somewhere along the lines of a 9600GT. The old G80s are power hungry and run hot, so having another is not a good idea. Your PSU isn't enough for two anyways.
Use it as a physics card and get a good ATI card.
$280 - 5850
$200 - 4890
$150 - 5770
$140 - 4870 1GB
$125 - 4870 512mb
$110 - 4850 or 4770 or GTS250/9800GTX+
Or sell your old 8800GTS 640mb for about $40 or so on ebay.