Thats all up to you if you want another 5970 in your system. Second 5970 will give you smother play in game (fps) and higher benchmarks score for bragging rights. Just like what omgitzfatal said invest the $$$ somewhere else. If you want to get amazed buy an ssd hard drive you will get fasted boot times, apps and games open faster, etc.
As for RAM 6GB is plenty.
Logitech makes very good speakers
Blue Ray burner is a good investment that is if you are going to watch and burn blue ray disks.
Get better cooling and overclock your i7 950 air or wc.
If you choose to buy another 5970 make sure your psu can handle both of them in CF.