The only graphics card upgrade I can buy with a warranty is through the Apple store, an ATI Radeon HD 4870 on sale for a pricey £286.00

In the PC world you can pick up a similar card for more than half that.

In fact, you can buy an ATI Radeon HD 4890 for about £180 or a newer ATI Radeon HD 5770 for £136!

All you need is a different type of graphics card power plug and Zeus software to flash the card.

Apparently it works for most ATI graphics cards – uncertain about the 5770 though.