One of my systems has an OC'd Q9550, 4 GB RAM, a GTX260 - a card with slightly lower power requirements than yours, a Gigabyte EP45-UD3P motherboard, 3 hard drives and an optical, and a Soundblaster card all powered by a Corsair 750TX.
Running 3 instances of Prime95 to load the CPU and 3DMark06 to load the GPU, it pulls 375 watts from the wall as measured by my Kill-a-Watt meter. Figuring 80% PSU efficiency, the system pulls 300 watts from the PSU.
I figure a similar system with a 4870 would pull about 30 watts more. A good 500 watt PSU would be operating at around 65% of capacity.