Well, most laptops do not have an upgradable graphics card.
If you have a laptop that does, then it comes down to the best of the ones that your laptop mfg actually makes / provides.
AMD® Radeon™ HD 7970M 2GB GDDR5 - ETA April 29, 2013 $0.00
Dual NVIDIA® GeForce™ GTX 670MX's Total 6GB GDDR5 in SLI $150.00
NVIDIA® GeForce™ GTX 680M 4GB GDDR5 Memory $200.00
Dual AMD® Radeon™ HD 7970M's total 4GB GDDR5 in CrossFireX™ - ETA April 29, 2013 $350.00
Dual NVIDIA® GeForce™ GTX 680M's total 8GB GDDR5 in SLI! $850.00
NVIDIA® Quadro™ K5000M Graphics with 4GB GDDR5 Video Memory $1,800.00
aint happening. even assuming you can pick up the GPU from a OEM source, you will have to find a compatable chassis and motherboard. go either get a desktop or get a laptop built at a custom vendor