There's two ways you could go about it, one is to buy an "expandable" pre-built, like the Swiftech H220X/H240X or the CoolerMaster Glacer 240 etc.. these are all in ones that come with the cpu block and radiator/pump etc.. but you would still be "building" part of the loop yourself, you would need to drain the fluid by opening it up, then buy some more tubing, fittings, and a waterblock for your GPU. The cheapest way would be to get what's called a "universal" GPU waterblock, they go for about $50-60 rather than $120-130 for full cover ones. Then you would need a small pack of vrm/mosfet heatsinks that usually come with some adhesive glue to attach them to your vrm chips since the universal waterblock doesn't cool those parts. The heatsinks are cheap, like $20 for a pack that would have 10-20 small sinks + adhesive to cool a couple GPU's. So you would run the original tubing and CPU block/pump on the pre-built to your cpu, but instead of leading back to the radiator, run the 2nd length of tube to your GPU waterblock, then use the tubing you buy extra of, to go back from the GPU waterblock to the radiator. Make sure your fittings are the right size to fit the AIO cpu cooler though.
The 2nd way to do this would be to get one of the NZXT Kraken or the new (and better imo) Corsair HG10 brackets, you take the cooler off your card, attach the bracket to the pcb, and then attach the fan etc.. and then take the expandable pre-built CPU cooler like the swiftech H220/H220X etc.. and the tubing/fittings like i mentioned and do the same thing except this way you are attaching the pre-built cooler to your GPU not the CPU, then you are going to want to buy a CPU water block and run the extra tubing/fittings to the CPU. So quite similar to the other way, except your attaching the swiftech cooler to your GPU and buying a $60 waterblock for the CPU rather than the other way around. To get a full custom water loop will cost a lot more than this, so it's not cheap.
A good parts list is like this
Method 1
Swiftech H220X $140
EK VGA Supremacy Universal GPU Waterblock or EK Thermosphere (looks much better, but a bit more expensive) ~$55
EK Backplate for your graphic card ~$35 (if there is one, not sure if your card has a backplate available. It's optional though, just looks better and protects the card from leaks)
Masterkleer Tubing $10 (real cheap, but good quality. $1 a foot at performance-pcs.com) Note that you will need the tube size to match the size of the fittings on the Swiftech cooler.
EK Compression fittings ~$5 each, (you'll need 2 of them. Your Swiftech cooler will come with two barb fittings already on it, so you just need fittings for the GPU)
Method 2
Swiftech H220X (attach to GPU not CPU) $140
Corsair HG10 or NZXT Kraken water cooling bracket ~$35-40
Masterkleer tubing $10
EK Compression fittings $5 each (buy 2)
EK Supremacy MX CPU waterblock ~$50