I'd stick it out with your present card, and see hpw the CrossFire stuff pans out. The downside to SLI, as I'm sure you all know, is that you need identical cards, and not a huge number of games support it. I'm sure something similar could be said in regards to Crossfire, I like the mix and match scheme they intend to run, but is there any point?
How long will it take games to catch up to CrossFire/SLI sufficiently for it to be of use?
I digress. I've not had much experiance with nVidia cards, I always found they overheated like bastards and stayed away. My preference would be a high end X1800, but, with most of the cards out now, you can hit the same stock X1800 speeds with a little tweaking.. so.. It depends on how much you want to be disappointed by the X1800 series. Personally, I'd stick with what you have, invest in some good coolers, and wait for Crossfire/SLI to really take off before I did anything.
The arguments about SM 3.0 are a little.. Eh. I can't see it taking off like SM 1.0/2.0 did, so.. It depends how much you have at yor disposal money wise.
g3pro makes a good point, though. Might be better waiting for the mid-season stuff. Graphics cards are a lot like iPods in some respects. Unless you buy them within a week of release, they'll be seriously out of date and updated before you've even got a decent leg out of the present piece of kit.