525w PSU running a 5870. Impressive. Well if we stay within the TDP of a 5870 that would leave you with being able to choose, an R270X, or R270 being top end.
Yes its really easy to change out a graphics card. Open case, remove screws from the back of the card thats holding it down, place finger on the PCI-E Slot holder to push it back or forward depending on the design of the board, Then pull card straight out.
However though if you are willing to replace that 525W unit. You could pick up say a Corsair TX650M , And pick up an R280X, Or Radeon 7970.(same card just rebrands)
I5 3570K@4~5.0ghz: Gigabyte GA-Z77X-UD3H:Corsair Hydro H110 :16GB Corsair Vengeance DDR-3 1600: x2 XFX Radeon 7950 @ 1100/1575 : Corsair AX 860 PSU: x1 WD Black 1TB HDD's: 90GB Agility 3: 960GB SSD : Creative Labs X-FI:Corsair 500R Arctic White Edition:3 22inch Monitors Eyefinity @ 5294x 1050 : Alienware Tact-X Keyboard: Razer Naga :Corsair Vengence 1500 Headset: Corsair SP2500 Speakers