Best Upgrade For My XPS8900 For Valve VR

I'm thinking about upgrading my XPS8900 to handle Valve VR Index for my birthday. What would be my best option-

1-New video card (and which one) or

2-New video card and power supply.

I'm looking to get a good deal, so I'm aiming at the middle of the range between "most powerful" and "meeting base specs at the lowest cost".

Thank you.