Without knowing what you want to do, that question is just about impossible to answer. And frankly I'm a bit confused about asking what is the cheapest GFX card to use w/ a $1500 monitor.
Those two monitors are built w/ very expensive IPS panels.....excellent choices for Photo buffs and GFX professionals requiring accurate color. If that's not you, then I'd buy a TN monitor and improve. If the goal is gaming, and I assume it is given Dell advertised that unit as a "gaming laptop", the card is way underpowered. Looking here:
http://www.tomshardware.com/reviews/radeon-hd-6950-1gb-geforce-gtx-560-ti-gaming-graphics-card,2857-7.html
It's listed in Tier 19. It certainly will be over matched with any of today's current games.
But, since the Dell XPS M1730 is a laptop, and "It's a Dell", I don't see that you have nay option but to remain w/what you have.
If I was going to spend $1500, and it must involve a laptop, it would be a custom built Clevo P170HM where you do have an option of updating the GFX at some point.
http://forum.notebookreview.com/sager-clevo/91510-clevo-guide-v2-0-faq-reseller-info.html
http://www.pro-star.com/index.cfm?mainpage=product&filter=7