Personally i'd rather have one card than two. Less heat, power usage, cables, more games support single cards than crossfired/sli'd ones, etc. This doesn't necessarily mean it's a bad idea to get a second card though, because you can save the second for when the first one goes out. Just have it ready and waiting on standby in case of emergency.
Yeah usually I prefer to have one card too for the same reasons. Tempted to get another 5870 though.
Anyway, my friend likes my case and I don't really like it any more, so I'll likely sell it to him with the PSU already installed and then I'll most likely end up getting myself a new case and power supply and I'd stick with the 5870 for a bit longer and then consider either getting another 5870 and Crossfiring or just getting something else and putting my 5870 in the C2Q rig and replace the 9800 GT which is in there.
He may not actually end up buying my case though, he may find something else, who knows, it's a little way off yet.