Which video card for a dual monitor Vostro 460?

It depends on the upgrade costs.  Sometimes they are less, but a lot of times they are much more than the same card you can buy at retail.  If I pull up the Vostro 460, the default shows only the Intel HD 2000 integrated graphics and they want $195 for an HD 6670 and you can get them for around $90 at retail.  If the default card is the GT 420, you are probably paying for that in some way.   The GT 420 is an OEM only card and not sold at retail.  It is a lower-end card and the new HD 6670 which is replacing the HD 5670 would definitely be better.  

I prefer retail cards since you have more choices to choose from to get just what you want if you have some experience.  On the downside, you have to go through the effort of physically installing the card and then installing the driver and hope that everything works as it should.   For me, it's a no brainer and I would go with the HD 6670 at retail based on the upgrade costs I have seen.