The PSU is Coolermaster extreme power 460W.
My budget's approx $300-400. Maybe able to push it to $450 max.
Yeah, upgrade the PSU. Coolermaster PSUs aren't really good.
For $400, I'd suggest you get the HD 7870 like Sunius said. But for 1440X900, that would be an overkill.
What are the rest of your specs?
Do you have a PC? Or are you building another one?
The CPU is quite as well important, and it'll help tell us whether the new GPU may be a bottleneck or not.