GPU & PSU Help

[squijee]

Hey so I'm looking to upgrade my graphic card this year but idk much about this cuz i never done it before.

If you need more info just let me know.

Here's what i have now,
it's a Dell XPS 8300
CPU - Intel Core i7 2600 @ 3.40GHz
RAM - 16.0 GB Dual-Channel DDR3 @ 662MHz (9-9-9-24)
Motherboard - Dell Inc. 0Y2MRG (CPU 1)
GPU - Radeon HD 6870
HDD - 2TB
Windows 7 Home Premium

the power supply i believe is only 460w and that's where the problem is if i upgrade the graphic card.

My goal is to be able to run everything maxed out and try to future proof myself.

I was looking at power supply and this one seemed like a good one link
is modular power supply a better choice?

and i was hopping for suggesting for graphics card?
I'm not sure about a price range but prob somewhere around $400 for GPU