There is only one card in the 3600 series, and that's the 3650. Of course, that's about as fast as the cheapo 6450 so yeah time for an upgrade
. I would say a 6870 would be a good choice, but go with a 6850 if you only have one PCI-E connectorhttp://www.guru3d.com/article/radeon-hd-6850-6870-review/10
For reference, a single 5870 in my system (with my CPU at stock clocks mind you) pulled just over 400W from the wall with furmark and 3 threads of Prime 95. Such is an unrealistic scenario for every day usage but it is a worst case. Even so, remember that the closer you push your PSU to it's rated wattage the hotter it generally runs and the shorter it's life.